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STRUCTURAL ENGINEERING 


FUNDAMENTAL PROPERTIES OF MATERIALS 


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STRUCTURAL ENGINEERING 


3 


a *ur, ~i 


FUNDAMENTAL 
PROPERTIES OF MATERIALS 


BY 

GEORGE FILLMORE SWAIN, LL.D. 

Gordon McKay Professor of Civil Engineering in Harvard University, formerly 
Chairman of the Boston Transit Commission and Consulting Engineer 
of the Massachusetts Railroad Commission, Past President Am. 

Soc. C. E., Member Inst. C. E., Am. Soc. M. E., Eng. 

Inst, of Canada, Am. Soc. for Testing Materials, 

Am. Ry. Eng. Assoc., Boston Soc. C. E. 

Am. Acad. Arts and Sciences, 

National Acad, of Sciences, 

Etc. 


First Eidtion 


McGRAW-HILL BOOK COMPANY, Inc. 
NEW YORK: 370 SEVENTH AVENUE 

LONDON: 0 & 8 BOUVERIE ST., E. C. 4 

1924 



Copyright, 1924 , by the 
McGraw-Hill Book Company, Inc. 

PRINTED IN THE UNITED STATES OF AMERICA 




• / 


r \ i'i t? q rt 


THE MAPLE PRESS COMPANY, 


YORK, PA. 


N(1V 18 74 

©CIA 807875 



PREFACE 


This volume is the second of the writer's work on Structural 
Engineering, which work is intended*to cover the parts of the 
subject most important for the engineer and especially for the 
engineering students in technical schools. 

It was originally intended that the present volume should 
form the second part of the volume on the Strength of Materials, 
with which it is closely connected; but in order to avoid making 
the first volume too bulky, this part is published separately. 
It does not deal in detail with processes of manufacture, which 
should be studied, as far as necessary, in the more specialized 
books herein referred to; but is intended to give the fundamental 
properties of the principal materials, which should be familiar to 
the engineer. Thus it does not overlap or replace the detailed 
works on Materials, but brings together logically, and it is hoped 
clearly, the fundamental properties, the constitution and physical 
structure, the importance and effect of various ingredients, the 
effect of different treatments, and the significance of the specifica¬ 
tions used to secure desired properties in the material to be 
employed. 

The book is intended to be used in connection with the previous 
volume on the Strength of Materials, and the needs of construct¬ 
ing engineers have been mainly considered. The point of view of 
the practical engineer has been emphasized throughout, as it was 
in the earlier volume. However incomplete this work, it may be 
of some interest to the student of the subject to see what portions 
of it the writer, who has done as much practical engineering work 
as teaching, considers most important for the constructing 
engineer to know. 

It is hoped that this volume will be found a suitable continua¬ 
tion of the first volume of the series. The third and fourth 
volumes, dealing respectively with the Theory and Design of 
Elementary Structures and with structures of more complicated 
character, are well under way. 

The writer's thanks are due to various sources for cuts, tables 


VI 


PREFACE 


and miscellaneous information. Acknowledgments have been 
made in the appropriate places. 

The writer is particularly indebted to his colleague, Professor 
Albert Sauveur, who has read the chapters dealing with metals; 
and also to his colleague, Professor G. P. Baxter, who has read 
the chapter dealing with cement and concrete. 

The writer will be grateful' to any reader who will call his 
attention to such typographical errors as may have escaped 
correction, or to any errors of statement into which the writer 
may have fallen. 

George Fillmore Swain. 

Harvard University, 

Cambridge, Massachusetts. 


CONTENTS 


Paoe 

Preface . v 

Chapter 

I. Introduction. 1 

II. Wood. 4 

III. The Constitution, Heat Treatment, and Mechanical 

Treatment of Iron and Steel.39 

IV. Cast Iron. 69 

V. Wrought Iron. 80 

VI. Malleable Cast Iron. 85 

VII. Steel.88 

VIII. Alloy Steels. 108 

IX. Nonferrous Metals and Alloys.126 

X. Stone.134 

XI. Brick and Other Clay Products.143 

XII. Calcareous Cement and Concrete.153 

XIII. Corrosion of Metals—Paints and Varnishes.185 

Index.197 


vii 
















“Read not to contradict and confute, nor to 
believe and take for granted, nor to find talk 
and discourse, but to weigh and consider.” 

—Bacon. 


STRUCTURAL ENGINEERING 


FUNDAMENTAL PROPERTIES OF MATERIALS 

CHAPTER I 

INTRODUCTION 
MATERIALS USED IN STRUCTURES 

1. Importance of the Subject. —The structural engineer, 
being a designer and constructor, must use materials. He must, 
therefore, be familiar with their methods of manufacture, proper¬ 
ties, and strength; otherwise, he cannot intelligently select 
proper materials for his purposes, understand the reasons for cur¬ 
rent practice or specifications, frame his own specifications intel¬ 
ligently if they have not been standardized, or test the material 
so as to satisfy himself that it meets his requirements. 

The subject is a large one. A man may spend his life in a 
study of one of the materials alone, like wood, concrete, or steel, 
if he goes into it thoroughly. It will not be attempted here to 
treat the subject extensively; but it is thought desirable to give a 
short statement of the most important fundamental facts and 
principles, so that the reader may understand the reasons for cer¬ 
tain designs and specifications. References will also be given to 
works on the subject which the structural engineer should study. 

2. References. —There are exhaustive works on each of the 
principal materials used, and some works which discuss all of 
them in a thorough manner. Among these works of a general 
character may be mentioned the following: 

(1) Johnson’s “Materials of Construction,” fifth edition, 
rewritten by M. 0. Withey and James Aston. This is a wonder¬ 
fully comprehensive work, treating of the manufacture, properties, 
and strength of most of the materials of construction. Published 
by John Wiley & Sons, Inc., 1919. 

1 



2 


STRUCTURAL ENGINEERING 


(2) Mills: “Materials of Construction/’ second edition, edited 
by H. W. Hayward. This is a briefer but excellent work treat¬ 
ing of all usual materials. Published by John Wiley & Sons, 
Inc., 1922. 

(3) Upton: “The Structure and Properties of the More 
Common Materials of Construction.” This is a book of differ¬ 
ent character from the first two, dealing almost entirely with 
metals, with an excellent short chapter on cement. It does not 
treat methods of manufacture, but discusses clearly and 
thoroughly the properties and the behaviour under tests, and the 
structure and treatment of steel. Published by John Wiley & 
Sons, Inc., 191G. 

(4) “The Metallurgy of Iron and Steel,” by Bradley 
Stoughton, is an excellent work, with many references for more 
detailed study. It should be mastered by the engineer before 
studying the other works dealing with all kinds of materials. 
McGraw-Hill Book Co. Inc., 3rd. ed., 1923. 

(5) Moore: “Materials of Engineering,” is a smaller book, 
giving an excellent brief treatment of the subject. McGraw-Hill 
Book Co. Inc., 1922. 

All of these works should be read by the structural engineer. 

Other excellent books might be mentioned, such as: I 

(6) “The Making, Shaping and Treating of Steel,” by J. M. 
Camp and C. B. Francis, published by the Carnegie Steel Co., 
1919. 

(7) Rosenhain: “Elements of Physical Metallurgy.” Constable 
& Co. Ltd., 2nd. ed., 1922. 

(8) Unwin: “The Testing of Materials of Construction.” 
Longmans, Green & Co., 3rd. ed., 1910. 

(9) Durand-Claye: “Chimie appliquee £ Part de l’ingenieur.” 
Baudry, 1897. 

(10) Martens: “Handbook of Testing Materials,” translated 
by Gus C. Henning, 1899, John Wiley & Sons. 

(11) Batson and Hyde: “Mechanical Testing,” E. P. Dutton 
& Co., 1922. 

3. Classification. -The materials used may be classified as 
follows: 


INTRODUCTION 


3 


Organic; wood 


Metallic; 


Inorganic; 


Non-metallic; 


Iron 


Cast iron 
Wrought iron 
Malleable iron 
Steel 

Alloy steels 


Other metals 


Stone 

Clay products 


Copper 

Lead 

Aluminum 

Zinc 

etc. 

Brick 

Terra cotta 


Cement 

Concrete 

Paints and preservatives 
(partly organic) 


4. Specifications for materials have been largely standardized, 
by the American Society for Testing Materials (A.S.T.M.), the 
American Concrete Institute (A.C.I.), the American Society of 
Civil Engineers (A.S.C.E.), etc.; and structural engineers gener¬ 
ally rely on the standard specifications, and employ a testing 
engineer or firm to supervise the tests. 

5. Materials of the same kind vary greatly in strength and 
other properties, depending on chemical composition, physical 
condition, and the treatment to which they have been subjected, 
such as seasoning in the case of wood, and mechanical or heat 
treatment in the case of metals. The reader of this part of the 
present work will doubtless be impressed by the inherent uncer¬ 
tainties surrounding the subject, and the wide variations in 
strength of material of the same kind. This should only empha¬ 
size the importance of the subject, and that the engineer should 
know the kind of material he wants and how to get it. Good 
design without proper material may go for naught. 








CHAPTER II 


WOOD 

1. References.—There are many works dealing with wood, of 
which only a few will be mentioned, which should be in the library 
of the structural engineer. 

(1) Snow: “Wood and other Organic Structural Materials.” This is an 
excellent and comprehensive work, and contains a very complete biblio¬ 
graphy. Published by McGraw-Hill Book Co., Inc. 

(2) Betts: “Timber, Its Strength, Seasoning, and Grading.” This book, 
by an officer of the U. S. Forest Service, is also excellent, dealing largely with 
the strength of timber. Published by McGraw-Hill Book Co., Inc. 

(3) The publications of the U. S. Forest Service treat of every phase of the 
subject. Snow’s bibliography covers them completely. 

(4) Hough: “Handbook of the Trees of the Northern States and Canada.” 
This is an excellent reference book, profusely illustrated. 

Both Johnson and Mills have excellent chapters on timber. 

2. Wood is the solid part of trees. The term ‘'wood” includes 
all woody tissue, even that which is not suitable for construction. 
“Wood that is suitable, although not necessarily ready, for con¬ 
struction, is ‘timber’; and wood that is not only suitable, but 
also ready for construction, is ‘lumber.’ The word timber may 
thus include living trees in the forest, as well as logs and shaped 
pieces; whereas, lumber refers only to boards, planks, beams, and 
other sawn pieces of limited sizes.” 

Wood is a fibrous material, consisting generally of partly 
interlaced cells or hollow tubes running lengthwise of the trunk 
or branch, more or less permeated by water, resins, coloring 
matter, etc. 

For the engineer in the temperate zone there are but two 
families of trees to be considered, namely, the Coniferae , often 
known as softwoods or evergreens, and the Dicotyledons , generally 
known as hardwoods or deciduous trees. 

The coniferous trees comprise the pines, spruces, firs, and 
cedars, and are the principal source of wood for engineering con¬ 
struction. Their seeds are naked, borne in cones, and the leaves 
are needle-shaped, though in some cases broader than this name 

4 


WOOD 


5 


would indicate. While often called softwoods, their wood is 
sometimes hard—harder than that of some of the so-called hard¬ 
woods—and the leaves of some drop off each year, and so they are 
not all evergreens. 

The other family might best be called broadleaf trees, 1 for the 
wood of some is soft, and the leaves of some are persistent. To 
this family belong the oaks, beeches, birches, maples, hickories, 
etc. Their seeds are enclosed in seed vessels. 

3. Both of these kinds of trees grow by the addition, each year, 
of an outside layer or ring of cells, between the old wood and the 
bark, and by a budding or telescopic extension at the ends of the 
limbs. A cross-section plainly shows these annual rings, sur¬ 
rounding the pitch center, and by counting them the age of the 
tree may be estimated. 2 The annual rings vary in thickness, 
and are not always uniform in the same ring. There are also 
horizontal cells, forming what are called medullary or pith rays, 
which are seen in a section, radiating from the center. A nail 
driven into a trunk remains at the same height as the tree grows, 
but it may be covered by new wood. 

The cells toward the outside of the trunk are more or less filled 
with sap, which passes upward from the roots through the outer 
layers to the foliage, is there changed, and passes down through 
the inner bark to the growing parts. The sap circulates during 
most of the year. The outer or newer part of the trunk is there¬ 
fore called sapwood , and the inner part heartwood, which is modi¬ 
fied sapwood. In the heartwood, the cells become filled with 
gums and inert materials, and it is heavier when dry and of a 
different and darker color than sapwood. 3 It takes no part in 
the processes of growth, and is thus virtually dead. The tree 
can survive even if the heartwood is largely decayed. In some 
trees the section appears to be almost wholly heartwood and the 
change of sapwood to heartwood progresses rapidly. In other 
trees the change is slow. 

Each annual ring consists of springwood, which grows rapidly 
in the spring, and summerwood, which grows more slowly and is 
darker colored, and easily distinguished from the spring wood of 

1 Snow. 

2 Palms, bamboos, and some other tropical plants are not outside growers 
(Exogens), but inside growers (Endogens), the new woody fiber being irreg¬ 
ularly intermingled with the old. 

3 When green, the sapwood contains more water than the heartwood, and 
is heavier. 


STRUCTURAL ENGINEERING 


G 

the succeeding year. The greater the proportion of summer- 
wood, the greater the weight and strength. 

Heartwood is generally preferred to sapwood, and is considered 
stronger. It is no doubt more resistant to decay than sapwood, 
because it is more penetrated by gums and resins, but as regards 
strength the Forest Service states 1 that, other things equal, 
sapwood is as strong as heartwood, except from old, overmature 
trees. Owing to the gums and resins in heartwood, it is less 
easily treated with preservatives. 

4. Moisture. 2 —Wood contains considerable moisture when 
freshly cut, and as it dries the moisture evaporates and the wood 
shrinks. Aside from the water which is chemically combined 
with the woody fiber, the moisture in wood is of two kinds; (1) 
that which is contained in the open spaces in the cells, the cells 
being hollow tubes; and (2) that which is absorbed by the cell 
walls, filling the smaller intermolecular spaces. The removal of 
moisture of the first kind has no effect on the physical properties 
of the timber except to diminish its weight; the removal of the 
second kind causes shrinkage and increases the strength, the cell 
walls becoming thinner. If we should start with wood as dry as 
possible, the strength would decrease as the moisture of the sec¬ 
ond kind increased, up to the point when the cell walls were 
saturated, after which increased moisture would cause no change 
in strength, and no more swelling. Kiln-dry wood contains about 
8 per cent of moisture in all species, and at the fiber saturation 
point the percentage is about as follows: 


Fiber Saturation Point op Wood 
Percentage of Moisture. (U. S. Forest Service , Circular 108) 


Kind of wood 

Percentage of moisture at fiber 
saturation point 

Longleaf pine. 

24-26 

29-35 

24-28 

23-26 

19-23 

25 

23 

26- 34 

27- 33 

Red spruce. 

Chestnut. 

Loblolly pine. 

White ash. 

Red gum. 

Douglas fir. 

Norway pine.. 

Tamarack. 



1 Bull. 108, on “Strength of Structural Timber.” 

2 U. S. Forest Service, Cir. 108 and Bull. 70. 
















WOOD 


7 


If, in addition to the cell walls being saturated, the cells them¬ 
selves are filled with water, or partly filled, the percentage of 
moisture will of course exceed these figures. 

Tests of strength of small specimens of woods, even of the same 
species, show large variations in the results. The main cause of 
these variations is differences in per cent of moisture. The 
strength when the cell walls are saturated may be only one-third 
to one-fourth of the strength when the wood is absolutely dry. 1 
In order to make results comparable, there should be a standard 
per cent of moisture. Stiffness is also reduced by moisture, but 
not so much as strength. 

Besides influencing strength, stiffness, and weight, the removal 
of moisture causes shrinkage, as already mentioned, and often 
produces defects. The shrinkage longitudinally with the grain is 
.so small as to be negligible, only a few tenths of 1 per cent. The 
transverse shrinkage is less in a radial direction than tangentially 



Fig. 1 .—Effect of shrinkage of timber. 


along the annual rings. This is partly due to the structure, and 
also to the fact that the outer layers contain the most moisture 
in a green state, as the sap circulates mostly in these layers. 
The moisture decreases towards the heart, and there is much less 
in the heartwood than in the sap wood. There may be little or 
none in the heartwood, depending on the kind of wood. Longleaf 
pine has very little moisture in the heartwood, where the cells 
are filled with gums, pigments, and resins, while the green 
sapwood is very wet. The shrinkage tangential to the annual 
rings is generally about twice that in a radial direction. The 
actual amount of shrinkage, from the green to the oven-dry 
condition, varies from about 2 to 8 per cent radially (for the 
conifers 3 or 4 per cent), and from about 5 to 15 per cent tangen¬ 
tially (for the conifers 6 to 8 per cent). A log cut into four parts 
by two cuts at right angles through the pith will have the four 
right angles changed to acute angles by the shrinkage; and a 
rectangular plank, cut tangentially, will become curved, convex 
1 Betts, pp. 28-29. 




8 


STRUCTURAL ENGINEERING 


toward the heart (Fig. 1); if the grain is not straight, it may 
also become twisted, or one end of the board may curve more 
than the other; if the grain is spiral, the twist may be greater. 

The cell walls are thicker in summerwood than in springwood, 
and the former almost always shrinks more than the latter. For 
the same reason, dense wood shrinks more than light wood of the 
same species. J. A. Newlin computes that the per cent of radial 
shrinkage is 9.5 times the specific gravity, and of tangential 
shrinkage 16.5 times the specific gravity. 

5. The Unequal Shrinkage of Wood Produces Internal 
Stresses. —The outer layers, containing the most moisture, dry 
first and tend to shrink, but their shrinkage is opposed by the 
inner layers which are still moist. This produces a circumferen¬ 
tial tension in the outer layers, and a radial compression between 
the outer and inner layers. As the tensile strength circum¬ 
ferentially is very small (see Art. 11) radial cracks, called 
“checks,” are often produced, extending inward radially from the 
outside (Fig. 1). If these relieve the internal stress, then as the 
inner layers shrink they tend to separate from the outside layers, 
and “shakes” or cracks consisting of separation between the 
annual rings, may be formed, as well as internal radial checks, not 
visible on the surface. If the outside dries too rapidly and 
becomes set in a partially dry condition while the inside is still 
moist, “casehardening” is produced, and the interior, as it dries, 
will tend to produce shakes and inside checks. The ends of a 
stick dry more quickly than the parts toward the center of the 
length, and this^often produces radial checks at the ends. If a 
plank is cut just to one side of the pith, but near it, the effect of 
shrinkage and wear is often to cause warping, and the inner 
layers to peel off; such a plank should be laid with the heart 
side down. 

6. Seasoning 1 is drying the wood, and may be done by piling 
it in the open air, or by drying it in an oven. In air seasoning, 
the wood should be piled so that the air will circulate around it, 
and so that rainwater will be quickly drained off. Seasoning 
should be done in a manner to prevent, as far as possible, the 
formation of checks and shakes. It should not be dried too 
rapidly, but as uniformly as possible. The moisture should not 
be evaporated from the outside faster than it can be brought to 
the outside from the interior. 

1 See Betts, chap. V. 


WOOD 


9 


Air-dried wood contains about 12 per cent of moisture; kiln- 
dried wood 8 per cent or less, depending on circumstances. 
Seasoned timber should be used for all interior work, such as 
floors, where shrinkage would produce objectionable cracks. 

7. Defects of Timber. —These are knots, checks, shakes, wane, 
rot, pitch pockets, cross-grain. Scarcely any timber, except 
small pieces, is free from defects, and it is therefore necessary to 
classify them and to specify how far they are allowable. They 
have been defined as follows by the A.S.T.M. 

Knots are caused by growing branches, or by dead branches 
adhering to the trunk during growth. If a branch dies and drops 
off while the tree is small, there will be a small knot near the 
heart. Knots may be of various sizes and shapes, and may be 
sound, loose, rotten, encased, or pith. 

A pin-knot is a sound knot not over inch in average diameter. 

A standard knot is a sound knot Jlz to 13^ inches in average 
diameter; and a large knot is one over inches. 

A round knot is oval or circular; a spike knot one which is sawn 
in a lengthwise direction. 

A sound-knot is solid and as hard as the wood, and fixed in 
position; a loose knot is one not firmly fixed in place; a rotten knot 
is not as hard as the surrounding wood; a pith knot is sound, 
with a pith hole not over y± inch in diameter at the center; an 
encased knot is surrounded wholly or partly by bark or pitch. 

The continuity of the wood fibers is of course interrupted or 
deflected by a knot. 

Checks are radial cracks often caused by seasoning. A 
through check extends from one face to the opposite face. 

Shakes are separations between the annual rings, often called 
“ring shakes” or “cup shakes.” They may be due to seasoning, 
or to the stresses caused by bending and twisting by the wind. 
A through shake extends from one face to an adjoining or opposite 
face. Sometimes a radial check near the heart is called a heart 
shake. 

Checks and shakes may exist in the interior of a stick, invisible 
from the outside. If on the outside, they may be invisible when 
the timber is green. 

Wane is irregularity on the edge of a piece. 

Rot is any form of decay. 

A pitch pockets is an opening between the annual rings contain¬ 
ing pitch or sometimes bark; if not over inch wide it is small, 


10 


STRUCTURAL ENGINEERING 


if not over % inch wide or 3 inches long it is standard, if larger 
than this it is classed as large. 

Cross-grain is where the fibers are not parallel to the edges. 
It may be due to the fact that the piece was not sawed parallel 
to the annual rings, so that the latter run diagonally across the 
stick. Spiral grain is caused by the fibers growing spirally 
around the pith instead of parallel to it. 

8. Grading Rules and Specifications. 1 —The manufacturers of 
the various kinds of lumber, through their associations, have 
formulated so-called “grading rules,” which divide lumber into 
various grades, and specify what defects are allowable in each 
grade. The engineer who writes a specification desires material 
of a quality conformable to his needs and to the stresses which he 
has deemed it proper to allow in his designs. He must, therefore, 
be familiar with the grading rules, in order to know which of the 
grades will satisfy his requirements. There is at present much 
confusion as to these rules, owing to the large number of lumber 
associations, railroad companies, and engineering societies which 
have formulated them. A proper grading rule and specification 
should be such that no dangerous piece would be accepted under 
it, and yet such that no piece would be excluded that really be¬ 
longs in the grade specified. Grading rules and specifications 
should go together, but the former are prepared as a rule by the 
manufacturers and the latter by the users. Perfect timber 
cannot be obtained except at prohibitive cost, and many defects 
are quite innocuous if the timber is properly used. The grade 
necessary, and the defects allowable, will depend upon circum¬ 
stances and the use to be made of the lumber. Many grading 
rules do not properly classify timbers according to structural 
strength, though they may be sufficient for woodworking indus¬ 
tries or where appearance is the main element. 

The reader is referred to the authorities quoted for full 
information as to grading rules. Betts gives a number of speci¬ 
fications, from which the following are taken: 

CHESAPEAKE AND OHIO RAILWAY COMPANY 
SPECIFICATIONS FOR LONGLEAF YELLOW PINE 

All lumber must be cut from living timber of good quality, and must 
be free from splits, shakes, loose or decayed knots, or defects which impair 

1 See Betts, Snow, or Johnson. Also U. S. Forest Service Bull 71, 
“Rules and Specifications for Grading of Lumber.” 


WOOD 


11 


its durability. It shall be well manufactured to the size ordered, and 
must be of longleaf virgin pine; no shortleaf yellow pine, bull pine, or 
loblolly pine will be accepted under this specification. 

Y.P. 1 1 

Bridge guard rails, platform joists, signal masts, bumper posts, mail- 
crane posts, trestle posts, cattle guards, semaphore posts, sills and 
braces, watertank frames, and dock timbers. 

All square lumber shall show two-thirds heart on two sides and not 
less than one-half heart on the other two sides. Other sizes shall show 
two-thirds heart on faces, and show heart on two-thirds of the length on 
edges, excepting where the width exceeds the thickness by 3 inches or 
over; then it shall show heart on the edges for one-half the length. 

YP. 2 

Bridge cross-ties, bridge stringers, watertank joists, scale timbers, 
and trestle caps. 

On all square sizes the sap on each corner shall not exceed one-sixth 
the width of the face; when the width does not exceed the thickness by 
3 inches to show one-half the heart on narrow face the entire length; 
exceeding 3 inches to show heart on narrow face the entire length; sap 
on wide faces to be measured as on square sizes. 

Y.P. 3 

Boards and plank for flooring. 

Boards and plank 9 inches and under wide to have at least one heart 
face and two-thirds heart on opposite side; boards and planks over 9 
inches to have not less than two-thirds heart on both sides. 

BUILDING CODE RECOMMENDED BY THE NATIONAL BOARD OF 
FIRE UNDERWRITERS, NEW YORK, 1915 

Rules for grading structural timbers of southern yellow 
pine, prepared in cooperation with the United 
States Forest Service 

Grade I 2 

(1) Requirements for Quality of Timber Based Upon Soundness and 
Density.— (a) Soundness.—Shall contain only sound wood. 

( b ) Density, as indicated by number of rings and proportion of 
summerwood.—Shall show on the cross-section an average of not less 
than one-third summerwood, measured over the third, fourth, and fifth 

1 Y.P. = Yellow pine. 

2 This is practically the same as the ‘'Select Structural” grade adopted 
by the Southern Pine Association. 


12 


STRUCTURAL ENGINEERING 


inches on a radial line from the pith. Timber averaging less than six 
annual growth rings per inch shall show an average of not less than one- 
half summerwood. Contrast in color between summerwood and 
springwood shall be sharp. 

In cases where timbers do not contain the pith, and it is impossible 
to locate it with any degree of accuracy by curvature of the rings, the 
inspection shall be made over 3 inches of an approximately radial line, 
beginning at the edge nearest the pith. 

(2) Restrictions on Knots in Beams.—Sound knots over 1% inches in 
diameter, or knots over % inch in diameter which are insecurely attached 
to the surrounding wood, shall not be permitted in the middle half of the 
length of narrow or horizontal faces of beams; nor in the middle half of 


Vo/3 

Vol. 2 . 

Vol. 3 





Top View 



the length of the wide or vertical faces within a distance equal to one- 
fourth their width from the edges (see Fig. 2). 1 No knot shall be per¬ 
mitted within these areas whose diameter exceeds one-fourth the width 
of the face on which it appears. 

The aggregate diameter of all knots within the middle half of the 
length of any face, shall not exceed the width of that face. 

(3) Restrictions on Knots in Columns.—Sound knots having diam¬ 
eters greater than 4 inches or one-third the least dimension of a column, 
or knots over H inch in diameter which are insecurely attached to the 
surrounding wood, shall not be permitted. 

(4) Restrictions on Shakes and Checks in Beams.—Ring shakes shall 
not occupy at either end of a timber more than one-fourth the width of 
green material, nor more than one-third the width for seasoned material. 
Shakes shall not show on the faces of either green or seasoned timber. 

1 A beam may be divided into three volumes, as shown in Fig. 2. Knots 
occurring in vol. 1 are the worst, and may almost ruin the wood for strength 
as a beam. Knots in vol. 2 are less objectionable and in vol. 3 still less 
objectionable, though much depends on the character of the knot. 
















WOOD 


13 


Any combination of shakes and checks which would reduce the 
strength to a greater extent than the ring shakes here allowed, shall not 
be permitted. 

(5) Restrictions on Cross-grain in Beams.—Shall not have diagonal 
grain with slope greater than one in twenty within the middle half of 
the length of the beam. 

Grade II 

(6) Requirements for Quality.—Grade II includes timber rejected 
from Grade I on account of either (a) having less density than required 
for Grade I; or (6) having more serious defects than are allowed in 
Grade I. 

(a) Timber rejected from Grade I because of deficient density, will 
be accepted in Grade II provided it meets all the requirements of 
Grade I, except that in Rule 1, (6), the requirements for one-third 
summerwood in material having six rings and over per inch, shall be 
changed to one-fourth; and that the requirement of one-half summer- 
wood in material having less than six rings per inch, shall be changed to 
one-third. 

(b) Timber rejected from Grade I for excess defects will be accepted 
in Grade II, provided its density conforms to Rule 1, (6), and its defects 
are limited as follows: 

(7) Restrictions on Knots in Beams.—Sound knots over 3 inches in 
diameter or whose diameter exceeds one-half the width of the face on 
which they appear, or knots which are insecurely attached to the sur¬ 
rounding wood, whose diameter exceeds inches or one-fourth the 
width of the face on which they appear, shall not be permitted in the 
middle half of the length of narrow or horizontal faces of beams; nor in 
the middle half of the length of wide or vertical faces within a distance 
equal to one-fourth their width from the edges. 

The aggregate diameter of all knots within the middle half of the 
length of-any face shall not exceed twice the width of that face. 

(8) Restrictions on Knots in Columns.—Sound knots having diam¬ 
eters greater than 6 inches or one-half the least dimension of a column, 
or knots insecurely attached to the surrounding wood, and having diam¬ 
eters greater than 3 inches or one-fourth the least dimension of a column, 
shall not be permitted. 

(9) Restrictions on Shakes and Checks in Beams.—Ring shakes shall 
not occupy at either end of a timber more than one-third the width for 
green material, nor more than one-half the width for seasoned material. 

Any combination of shakes and checks which would reduce the 
strength to a greater extent than the ring shakes here allowed, shall not 
be permitted. 

The so-called “ Density Rule/’ which is the result of investiga¬ 
tions by the U. S. Forest Service and the A.S.T.M., and which was 


14 


STRUCTURAL ENGINEERING 


first definitely applied commercially in the inspection of a large 
shipment of yellow pine for the Panama Canal, is as follows: 

DENSITY RULE FOR GRADING SOUTHERN HARD PINE 

Dense Southern yellow pine shall show on either end an average of 
at least six annual rings per inch and at least one-third summerwood, or 
else the greater number of the rings shall show at least one-third summer- 
wood, all as measured over the third, fourth and fifth inches on a radial 
line from the pith. Wide-ringed material excluded by this rule will be 
acceptable provided that the amount of summerwood as above mea¬ 
sured shall be at least one-half. 

The contrast in color between summerwood and springwood shall 
be sharp, and the summerwood shall be dark in color, except in pieces 
having considerably above the minimum requirement for summerwood. 

In cases where timbers do not contain the pith and it is impossible to 
locate it with any degree of accuracy, the same inspection shall be made 
over 3 inches on an approximate radial line beginning at the edge nearest 
the pith in timbers over 3 inches in thickness, and on the second inch (on 
the piece) nearest to the pith in timbers 3 inches or less in thickness. 

In dimension material containing the pith, but not a 5-inch radial 
line, which is less than 2 by 8 inches in section or less than 8 inches in 
width, .that does not show over 16 square inches on the cross-section, 
the inspection shall apply to the second inch from the pith. In larger 
material that does not show a 5-inch radial line the inspection shall 
apply to the 3 inches farthest from the pith. 

Sound Southern yellow pine shall include pieces of Southern pine 
without any ring or summerwood requirement. 

The reader should also consult the following: 

Specifications of the A.S.T.M. for Yellow Pine Bridge and 
Trestle Timbers; and tentative specifications for Structural 
Douglas Fir. 

Specifications of the A.R.E.A. for Southern Yellow Pine Bridge 
and Trestle Timbers. 

Standard Specifications for Grades of Southern Yellow Pine 
Lumber, issued by the Georgia-Florida Saw Mill Association, 
1918. Standard Grading Specifications for Yard Lumber, as 
recommended by the Dept, of Agriculture: Department Circular 
296, Oct., 1923. 

Those ignorant of the subject may, and sometimes do, commit 
serious and ludicrous errors in framing specifications. The 
writer has heard of one specification which read “yellow pine 
furnished under this schedule must be entirely free from 


WOOD 


15 


heart”; and some that called for material “free from sap, 
knots, wane, and checks, and manufactured from untapped 
lumber,” a requirement probably impossible of fulfillment. 
Such specifications may be written knowing that they cannot be 
fulfilled, but with the hope that better lumber will result than 
under usual requirements; in other words, not expecting to 
get what is asked for. Such requirements are often disregarded 
by dealers, who furnish the usual stock. This demoralizes the 
situation, and often forces honest bidders to resort to the same 
disregard of the requirements. A specification should only ask 
for what it is practicable to get, and should he strictly lived up to. 
A bidder who bids on a certain specification hoping to furnish an 
inferior quality than is called for, and “puts it over,” demoralizes 
the situation. 

9. Kinds of Wood Used in Structures. 1 —While many different 
kinds of wood can be used for structures, the requirements of 
size, strength, durability, etc., are such that the woods used are 
almost exclusively conifers. Southern yellow or longleaf pine 
(.Pinus palustris Mill ) is the strongest, stiffest and best of the 
eastern woods. 2 It formerly covered most of the coastal plain 
of the southern Atlantic States, but has been largely cut away, 
and is now becoming scarce. White pine, shortleaf pine, spruce, 
and other conifers have been largely used in the east. 

The great future source of structural timber in the United 
States is in the forests of the Pacific Coast, where there are 
large stands of Douglas fir, western hemlock, redwood, and other 
woods. Of these, Douglas fir is the most suitable for structural 
purposes. 3 These trees grow to great heights, the tallest speci¬ 
men recorded being 380 feet high, and they furnish timbers of 
very large size. Logs that yield timbers 2 feet square and 100 
feet long are not uncommon. Western hemlock is also a useful 
timber, much better and stronger than eastern hemlock. Cali¬ 
fornia redwood grows very tall, up to 320 feet high and 35 feet 
in diameter. It is the oldest of trees, some of them being the 
oldest living things on earth. It offers unusual resistance to 
decay and to fire. Sitka spruce was much used during the 
war for airplane stock. It is light and soft, but strong for its 
weight. It grows to a height of 296 feet. 

1 See Snow, Chaps. V, VI, VII, for an excellent treatment. 

2 See Forest Service Cir. 164, “Properties and Uses of the Southern Pines.” 

3 See “Structural Timber Handbook on Pacific Coast Woods,” published 
by the West Coast Lumbermen's Association, Seattle. 


16 


STRUCTURAL ENGINEERING 


Oak, formerly much used in structures, has given way to the 
conifers, and is now mostly used for cabinet work. Elm is some¬ 
times used for piling, chestnut for railway ties. 

Greenheart, 1 a tree which grows in British Guiana and the 
West Indies, is stronger and stiffer than any North American 
wood. Its heartwood is very resistant to decay and to marine 
wood-borers, very hard and durable. It weighs about 72 pounds 
per cubic foot when green, and about 57.5 when oven dry. It is 
hard to work, and liable to split. It is much used for piles and 
lock gates in northern Europe, and was specified for the sills and 
fenders of lock gates for the Panama Canal. 



Fig. 3.—Relation of diameter of pith to diameter of second annual ring. 

If the point of intersection falls above the diagonal line in the diagram, the specimen 
is longleaf or m rare instances slash or pond pine. If it falls below the line, the specimen is 
not longleaf, but is shortleaf, loblolly, or some other minor southern pine. 

Eucalyptus is much used for structures in Australia. Some 
species of this tree are the tallest, though not the largest, known 
to man, growing to heights of over 400 feet. The wood is tough 
and durable. 

Bamboo is one of the strongest of all woods, and is much 
used in China and Japan. 

All the kinds of trees in common use for any purpose, and 
the properties of the woods, are well and fully described by Snow. 

10. It is often difficult to distinguish the different kinds of 
pine woods from one another. Only the expert can distinguish 
longleaf from shortleaf or loblolly pine. Forest Products Labora¬ 
tory Technical Note 141 gives a method, if the piece contains the 

1 See Armstrong, A. K., Engineering Record, Jan. 29 and Feb. 5, 1910. 







































































WOOD 


17 


pith. This pith, or small, dark core, averages larger in longleaf 
than in shortleaf or loblolly, being over 0.1 inch in diameter. 
If the pith in the shortleaf or loblolly is over 0.1 inch in diameter, 
the diameter of the second annual ring is larger than in longleaf. 
If the pith in the piece examined is about the size of the lead in 
an ordinary lead pencil, or smaller, the wood is not longleaf; 
if it is plainly over 0.1 inch in diameter, and the growth rings 
surrounding it very narrow, the wood is longleaf. If there is 
doubt, measure the average diameter of the pith, not including 
small projections, and the diameter of the second annual 
ring. Then refer to Fig. 3 and find the point corresponding to 
these measurements; if it lies above the diagonal straight line, 
the wood is longleaf; if below, it is shortleaf, loblolly, or some 
other of the southern pines. 

11. Strength of Timber. —Most experiments on the strength of 
timber, particularly those made before 1880, were made on small 
specimens, say 1 to 4 inches square, in section, of clear timber 
without defects. Such tests naturally showed greater strength 
than would result from tests on commercial sizes. Professor G. 
Lanza, of the Massachusetts Institute of Technology, was the 
first to show, about 1882, the inapplicability of the results ob¬ 
tained to commercial sizes for practical use and having usual 
defects, and to make and recommend tests on large specimens 
taken from ordinary stock. Such tests are the only safe guides 
for structural design. Tests on small selected pieces may give 
strengths twice as large as tests on commercial sizes, up to say 
6 by 12 inches or even 10 by 16 inches with spans up to 15 feet 
for transverse tests. 

On the other hand, tests on small, clear specimens, free from 
defects are often chosen, because such tests are the only ones 
that show the quality of the wood itself, and allow comparisons 
of the effect of moisture, density, etc. The quality of the wood 
is an element entirely independent of the presence of defects, 
which may or may not exist, and are very variable. 

The reader is cautioned against comparing values of strength 
from different sources. Some series of tests have been made 
without taking any account of moisture or reducing to a common 
standard. One of the greatest sources of error is in comparing 
things that are not comparable. 

Differences in strength of timber are due mainly to differences 
in (1) quality of wood, (2) defects, (3) moisture. 


18 


STRUCTURAL ENGINEERING 


The amount of moisture affects greatly the strength given by 
tests of small perfect specimens, as has been shown. Commercial 
sizes, such as are used in structures, are difficult to dry thoroughly; 
if exposed to the weather they would be wet anyway, at times; 
and the process of drying often produces defects which offset 
the increase of strength due to drying, as shown by small speci¬ 
mens. Hence, in structural work, in general, the design should he 
based on the strength values shown by tests of green timber. Season¬ 
ing cannot be depended on to increase the strength. 

The strength of dry timber depends upon (1) the quality of the 
wood itself, irrespective of defects, and (2) the defects existing, 
their character, number, location, and condition. The quality 
is quite accurately indicated by the dry weight. As between dif¬ 
ferent kinds of woods (dry), the heavier the wood the stronger 
it is; and as between different specimens of the same wood, the 
heavier piece will be the stronger (aside from defects). 1 

The U. S. Forest Products Laboratory suggests the following 
empirical formulae for the modulus of rupture in the flexure 
formula, irrespective of species; 

Dry timber; modulus of rupture = 26,200 X (specific 

gravity) 1 - 20 

Green timber; modulus of rupture = 18,500 X (specific 

gravity) 1 * 20 

These formulae were derived from tests on small, clear, straight¬ 
grained specimens. 

The relative weight, and therefore strength, of different pieces 
of the same species may be judged by the relative proportion of 
summerwood in the annual rings, summer wood being consider¬ 
ably denser and heavier than springwood. 

There is also a certain rate of growth which generally gives the 
greatest density and strength of a given species. This rate is 


stated as follows: 2 

Rings 

per 

Rings 

per 

Variety 

Inch Variety 

Inch 

Douglas fir. 

. 24 Tamarack. 

. 20 

Shortleaf pine. 

. 12 Norway pine. 

. 18 

Loblolly pine. 

. 6 Redwood. 

. 30 

Western larch. 

Western hemlock. 

. 18 Longleaf pine. 

. 14 

. 10 


1 An exception to this rule is found in the timber from the under side of 
leaning coniferous trees, known as “compression wood,” which has very 
wide rings, and while heavy, is low in strength. 

2 Beogs, p. 50. 











WOOD 


19 


Density, however, is the best reliance, and the “ Density Rule,” 
quoted in Art. 8, has been employed for grading southern yellow 
pine. 

Bleeding pine trees for turpentine does not affect the strength of 
the timber. The strength of longleaf pine is independent of the 
resin content. 

The results of tests of strength are very discordant, depending 
upon moisture, defects, sizes of specimens, and variability in the 
material itself. The conclusion to be drawn is that the factor 
of safety should be larger in timber than in more uniform 
materials, that special care should be used in selecting timber to 
be used in important structures, that no piece which is seriously 
defective should be used, and that timber pieces should be so 
placed that any defects that exist may do the least harm. 

Tensile Strength. —The ultimate tensile strength of timber 
along the grain is high, running in some woods up to over 30,000 
pounds per square inch. A failure by tension in this direction 
involves tearing across the fibers. It is difficult to make tension 
tests, because, even if the specimen is tapered down to a small 
section at the center, it will be apt to fail by shearing or pulling 
out, either at the ends where the pull is applied, or at the center 
if the grain is not exactly straight, as it seldom is. It is difficult 
to get a pure tensile fracture, and when obtained it is seldom a 
clean fracture, as in metals, but is much splintered. Tensile 
tests, however, are seldom made; and fortunately the tensile 
strength in structures is seldom or never a controlling element, 
because the strength of the connections of a tensile piece is less 
than the tensile strength of the piece itself. Its principal applica¬ 
tion is in the tension fibers of beams. Tensile strength is largely 
dependent on straightness of grain, knots, and the thickness of 
the cell walls. It is less dependent on moisture than the other 
properties are. 

The tensile strength perpendicular to the grain, which merely 
involves tearing the fibers from one another, either radially or 
tangential to the annual rings, is much less than the tensile 
strength along the grain, rarely exceeding 1,000 pounds per square 
inch, and in some woods running down to 200 pounds or less. 
Wood is seldom exposed to tension of this kind, except in some 
details which will later be pointed out. In the tension fibers of 
beams that are cross-grained, this kind of tension may exist. 
The tensile strength across a radial plane is generally less than 


20 


STRUCTURAL ENGINEERING 


across a plane tangent to the annual rings, especially in oaks and 
hardwoods with large medullary rays. 

Compressive Strength .—The compressive strength along the 
grain is considerable, sometimes 8,000 to 10,000 pounds per 
square inch for short pieces which do not bend. It depends 
upon the amount of wood fiber in the cross-section, the lateral 
adhesion of the fibers and their continuity. Failure occurs by 
crumpling of the cell walls, often showing itself by shearing along 



Fig. 4.—Failure of a short wooden block in compression. 


a plane inclined somewhat steeper than 45°. A long wooden 
column may fail by lateral deflection, and a column formula must 
be used. 

Figure 4 shows the failure of a short wooden column in compres¬ 
sion. 

The compressive strength across the grain involves crushing in 
the cell walls like compressing a hollow tube laterally. Failure, 
or fracture, does not occur under this kind of stress, and some¬ 
times there is a progressive yielding from the beginning; though 







WOOD 


21 


more often there is no appreciable yielding until a certain load is 
reached, after which the wood crushes continuously. The 



strength in this direction is assumed to be that at the elastic 
limit, or at some specified yielding. 



If W ood is in compression at an angle to the grain, as in Fig. 5, 
the allowable normal stress may be found as follows: let ab = 1 























22 


STRUCTURAL ENGINEERING 


inch, then be = sin a, ac = cos a) let the allowable stress along 
the grain be f g , and that across the grain f t . Then it seems 
reasonable that the surface ab could sustain a stress of ft - ac = 
ft cos a perpendicular to ac, and a stress of f g - be = f 0 - sin a per¬ 
pendicular to be. Then the total normal stress on ab, which is 
the stress intensity, since ab = 1 inch, is, 



Fig. 7.—Wood in compression parallel to grain. Specimen 6 X 6 X 24 inches. 

(Betts, v • 19.) 

de + df = ft cos 2 a + f g sin 2 a. 

Figure 6 gives the allowable stresses for various angles a, by 
this formula, namely 

f = ft cos 2 a +/„ sin 2 a. 

If the reader thinks that the surface ab could not safely sustain 
simultaneously the two forces f t cos a and f a sin a, then he may 
adopt some empirical rule more satisfying to his instinct. 

























WOOD 


23 


Figures 7 and 8 show stress-strain diagrams for wood in com¬ 
pression, taken from Forest Service Bull. 108. These show a 
quite well-defined elastic limit, but often the line is more curved, 
and the limit is not well defined. The initial portion of the dia¬ 
gram, however, is generally nearly straight, and the elastic limit 
can be approximated to as in Figs. 7 and 8. A wooden piece, 



in which the elastic limit is much below the ultimate breaking 
strength, will give warning before it fails, which is advantageous. 

Shearing Strength .—This is very different according as it is 
along or across the fibers. The latter, which involves shearing 
off a fiber crosswise, is large; but since this shear cannot exist 
without the simultaneous existence of a shear of equal intensity 
along the fiber, it is of no importance structurally. The shearing 
























24 


STRUCTURAL ENGINEERING 


strength along the fibers, which merely involves sliding one layer 
of fibers on another and overcoming the cohesion between them, 
is very small, and acts along a plane in which there may be a check 
or shake. This shearing strength will therefore be much greater 
in small selected specimens in which an area of sound wood a few 
inches square is sheared off, then it will be in large sticks, in 
which defects may exist. The values found by tests of small 
specimens are sometimes three or four times those found in beams 
which, under transverse loads, fail by longitudinal shearing at 
the end. 

Transverse Strength .—This is a combination of tension, com¬ 
pression and shearing, and as the tensile strength is greater 
than the compressive strength, failure will generally occur 
by compression or shearing, unless there are defects, such 
as knots or cross-grain, which cause it to fail by tension. The 
modulus of rupture, as computed, does not represent a stress, 
because it is computed on assumptions which are not valid up to 
rupture. It is a purely fictitious stress, and will always exceed 
the real compressive stress, for reasons that are explained in the 
chapter on Flexure, in Vol. I. Nevertheless, it serves as a basis 
of comparison, and, with a proper factor of safety, may be used 
in designing. 

The stress distribution in a wooden beam is not planar, due to 
lack of homogeneity and difference of elasticity in tension and in 
compression. The neutral axis in pure flexure does not go through 
the center of gravity. 

12. Effect of Defects on Strength. —Checks and shakes may 
obviously reduce or entirely destroy the resistance to shearing. 
Judgment and a knowledge of existing stresses are required to use 
properly timbers with serious checks or shakes. 

Cross-grain or spiral grain may be a serious defect in tension 
or bending, but less serious in compression or shearing. Cross¬ 
grain on the tension side of a beam may cause failure. In using 
Sitka spruce for airplanes, the specification required that no 
spiral-grained wood should be accepted with over 1 inch twist in 
a length of 20 inches. 

Knots may seriously impair or entirely destroy the strength in 
tension. If sound, they may do little harm in compression or 
shearing. If loose, they impair the strength in every way. A 
beam should, if possible, have no large knots or irregular grain 
on the tension side near the center of the span. A small knot in 


WOOD 25 

one location, as near the edge of the tension side of a beam, may 
be more injurious than a large knot on the compression side. 

13. Effect of Time on Strength of Timber. —The figures gen¬ 
erally, if not always, given for strength of timber represent the 
ultimate strength obtained by one application of the load. In 
almost all cases a smaller load, if allowed to remain for weeks or 
months, would ultimately cause failure. It is generally best to 
assume that the strength under a permanent load is not over one- 
half that given by a short-time test; and in computing deflection 
to assume the modulus of elasticity one-half the value given by 
short-time tests. 

14. Tables of Strength. —Table I gives data regarding the 
strength of some timbers, taken largely from more extensive 
tables given by Betts, and in Ball. 556, U. S. Forest Service. 
These figures are from tests on small, clear specimens, and there¬ 
fore give the strength independent of defects. 

Table II, from Betts gives figures for green structural timbers 
as compared with those for small clear specimens. 

Bulletin 556 of the Forest Service gives approximate figures 
for the percentage of change in the various,strength figures and 
other properties, due to changes in moisture and specific gravity. 

15. Weight. —Table I gives the weight per cubic foot of the 
different species. It is a curious fact that the weight of the wood 
fiber (cellulose) is almost the same for different species of North 
American woods, the specific gravity of all species being about 
1.6; so that no wood would float were it not for the air in the 
cells, walls, and intercellular spaces. “ Accurate determinations 
by the Forest Products Laboratory on seven species of wood, 
including both hardwood and coniferous species, showed a range 
of only about 4.5 per cent, in the density of the wood sub¬ 
stance.” 1 The greater the specific gravity of a given piece of dry 
wood, therefore, the more wood substance there is in it, and the 
greater the strength, normally. 

The lightest wood is Balsa, which grows in Central America and 
the West Indies, and weighs but 7 pounds per cubic foot. 2 The 
heaviest is black ironwood, which weighs about 81 pounds per 
cubic foot. 

It is common, for bridges, to specify that wood shall be assumed 
to weigh 4.5 pounds per foot board measure, or 54 pounds per 

1 Forest Service Bull. 556. p. 5. 

2 See Trans. A.S.C.E., May, 1916. 


Small, clear, green specimens. 2X2 inches, 28-inch span in bending. (From Betts and Forest Service Bull. 556) 


STRUCTURAL ENGINEERING 


Work in 

bending 

to elastic 

limit, 

inch 

pounds 

per cubic 

inch 

Goes ’t (X> gc o »o o a co oo 

O CO *C eeWOOOOONO *000*0 

OOO hhhOOhOO OOO 

Tension 
perpen¬ 
dicular 
to grain, 

pounds 

per 

square 

inch 

OOO • *000000 OOO 

O 00 CO • •NO'OCiWO (NO o 

Tf rfi • • h- <N CO <N CO <N <N <N N 

Shearing 
parallel 
to grain, 7 
pounds 
per 

square 

inch 

O‘0O<NNH00O5‘0C0O^ CO o 

'HHOWNClTf005C0 05H 0 0*0 

WHXC5l<Tt(0105NO00O r^GOOO 

1-H f-H rH rH 

Modulus 
of elas¬ 
ticity 6 in 
bending, 
pounds 
per 

square 

inch 

1,242,OOO 
1,540,000 
930,OOO 
2,792,000 
1,553,000 

1,346,000 

1,260,000 

1,576,000 

1,276,000 

1,618,000 

1,450,000 

1,073,000 

1,065,000 

1,215,000 

1,236,000 

1,060,000 

Compression 
perpendicular 
to grain at 
elastic limit, 5 
pounds per 
square inch 

r^OOCCTt<00(X><NCO*OTt<*0000*0 

O*0XC0^XX(MNXNHN^00N 

OrfCCCG(NOX»OCC*0^tCC*OCO^CO 

H rH rH 

Com¬ 
pression 
parallel 
to grain, 4 
pounds 
per 
square 
inch 

OOOO OOO O *00000000 
XONh(OCOOON(Oh(NOOOOC 5 
^^rt<a5NWCOO5C5W00NOC0^00 

COCONO^^COCON^CONt^NCON 

Bending 

modulus 

of 

rupture, 3 

pounds 

per 

square 

inch 

i 

*0000000010^000000 

COOOTfKOaOOHiOOHHONOO 

hn(OWWC5(NOO^(OC5COOCOhcO 

OOGO*OX(NOGONOXNION*ON^O 

H H rH 

Weight 8 in pounds 
per cubic foot 

Cl 

£ >> 

S’ 8 

WC0 050HHC«tOOO(OCONNC5 
*t ^ Cl C *0 »C’t CO CO ^ CO CJ Cl CJ CO 

r-b 

<~a 

^‘COCJCO^X^hCJOON^OOOOO 
TfTt4C0O»0»0^C0CC^C0 0^<N<NC0C0 

Green 

|OX‘OOI1*1*IOXIOOOC5X*ONH 

ic*o»or^ooococo»o*ocococo^T^ 

Locality 

Pa M Ind. 

Pa., Wis. 

Md., Tenn 
Demarara, S. A. 
Ohio 

Ohio 

La. 

Wash., Ore. 
Mont., Wyo. 
Miss., La., Fla. 
Ark., La. 

Wis. 

Cal. 

Tenn. 

Wis. 

Wash. 

Kind of wood 

Beech. 

Birch, yellow. 

Chestnut. 

Greenheart. 

Hickory, pig nut. 

Hickory, shag bark. 

Oak, white. 

Douglas fir. 

Douglas fir 9 . 

Pine, longleaf. 

Pine, shortleaf. 

Pine, white. 

Redwood. 

Spruce, red. 

Tamarack. 

Western hemlock. 


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Table II. —Average Strength of Green Structural Timbers with Ordinary Defects and Small Specimens 

without Defects 

(,From Betts) 


WOOD 



P 

cS 

a > 

rC 

02 


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28 


STRUCTURAL ENGINEERING 


cubic foot. Such wood is of course likely to be saturated with 
moisture. 

In view of all the foregoing, it must be recognized that timber 
is a very variable material. The student is advised to read, at 
this point, Art. 80 of “Structural Details” by Professor H. S. 
Jacoby, (John Wiley & Sons, Inc., 1910); and to consult the 
Proc. A.R.E.A. for 1909, pp. 543-565. 

16. Allowable Stresses.— In deciding upon working stresses, 
it is necessary to divide the ultimate by a factor of safety, which is 
variously taken from 4 to 8, and higher still for wood exposed to 
impact. The factor may also vary according to the kind of stress, 
and the relative importance of defects. 

In bending, a defect such as a knot in the tension side near the 
center, or a check near the neutral axis near one end, may make a 
stick nearly worthless, while its failure might be serious. A 
factor of 6 in flexure, for steady loads, applied to the ultimate 
given by short-time tests, which would really be a factor of 3, 
would not be too large (see Art. 13). This would give for long- 

leaf pine — = 1,440 pounds per square inch. A common 

figure for highway or railway bridges is 1,200 to 1,500. 

In compression along the grain a defect would not generally 
weaken a piece as much as in flexure, and here a factor of 5 might 
be justified. 

In compression across the grain, a defect would have still less 
influence, and a failure would often cause no serious results, as in 
bearing on wall plates of bridges. Moreover, there is no ultimate 
strength for this kind of stress, and the figures given in the table 
are for the elastic limit. Here the factor of safety may be small, 
and is sometimes taken as low as 1.5, or the allowed stress is % 
of the tabular figures, or for yellow pine 390 pounds per square 
inch. The factor is not generally taken as low as this, but 
is commonly from 2 to 2.5. 

In deciding on the factor of safety, it must be remembered 
that if, as stated above, the ultimate strength for a steady load is 
about one-half the value given by short-time tests, then the 
effect of a suddenly applied load (which is double that of a 
gradually applied load) is no greater than the effect of a steady 
load; that is, the allowance for impact may he much less for wooden 
structures than for metal structures. 



WOOD 


29 


The factor of safety should also depend upon the use, exposure, 
expected life, and other circumstances. 

The U. S. Forest Products Laboratory gives the following table 
(III) of working permissable stresses, under different conditions, 
which merits careful study. 1 

The Committee on Wooden Bridges and Trestles, of the 
A.R.E.A., recommends the following as safe working stresses 
(Table IV). 

In the “Specifications for Bridges and Subways” by Henry B. 
Seaman, 2 the following are the allowed static stresses on timber: 


Nature of stress 

White oak 

White 

pine 

Georgia 

pine 

Spruce 

Hemlock 

Tension, with grain. 

1,500 

1,000 

1,800 

1,200 

900 

Tension across grain. 

300 

75 

90 

75 


Compression, end bearing. 

2,100 

1,600 

2,400 

1,800 


Compression to 15 diameters. . . 

1,300 

1,000 

1,500 

1,200 

1,200 

Compression, columns 1 . 

1,600 

1,200 

1,800 

1,400 

1,400 

Compression across grain. 

750 

300 

500 

300 

200 

Shear across grain. 

1,500 

750 

1,900 

1,100 

900 

Shear with grain. 

300 

150 

200 

150 

150 

Bending, outer fiber. 

1,500 

1,000 

1,800 

1,000 

900 

Modulus of elasticity. 

1,100,000 

1,000,000 

1,700,000 

1,200,000 

900,000 


J Numerator in the formula ^ ™—» but not above the values given in previous 

A i . _ 1 _ 

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line for 15 diameters. 

I = length of column in inches. 
d = least diameter in inches. 


Committee D-7 of the A.S.T.M., on Timber, makes the following 
remarks, on pages 389-390 of the Proceedings of the American 
Society for Testing Materials , 1921. 

Working Stresses in Structural Timber—The committee offers the 
following comments in regard to working stresses in structural timber: 

(1) It is well determined that the strength of a particular piece of 
timber is in a measure determined by the condition under which it is 
used. From the tests it is seen that increase in moisture decreases the 
strength of timber, therefore it must be determined, first, in designing 
a particular structure, what the moisture conditions are. 

(2) It is also well determined by the tests that resistance to suddenly 
applied loads is much greater than to slowly applied or constant loading; 
therefore the condition of loading will affect the amount of allowable 
stress. 

1 Circular on Grading Rules and Working Stresses, 1920. 

2 Trans. A.S.C.E., vol. LXXV, p. 313, 1912. 

























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WOOD 


31 


(3) Warning of failure of a piece of timber in a structure is usually- 
given a considerable time in advance of actual failure; therefore, when 
efficient inspection is had this feature gives an element of safety in older 
structures. 

While there is no well-defined “elastic limit” in timber tests, there is 
in general a region where the proportionality of stress to strain ceases 
to be constant. It is therefore well to keep stresses well within this 
limit. Timber, however, does recover from high stresses; therefore its 
resistance to quick loading. 

From the above it will be seen that in order to have a rational design 
it is necessary to state working stresses of different amounts for different 
kinds of loadings and exposure of timber. As an example, for dense 
structural yellow pine, the maximum working stress will be 1,100 pounds 
per square inch. This is for constant loading and for submerged loca¬ 
tions where the timber is constantly wet. In locations in the weather, 
such as bridges, the allowable working stress for constant loading is 
1,400 pounds per square inch. Under cover, where the timber is always 
dry, the allowable working stress for constant loading is 1,600 pounds 
per square inch. 

From the tests, it is determined that the resistance of timber is approx¬ 
imately proportional to the speed of loading. For constant loading, 
the stresses above given are proper, but for sudden loading, resulting in 
100 per cent impact, the successive loadings being far enough apart so 
as to allow reasonable recovery of the timber, the allowable stresses may 
be doubled (not to exceed 2,800 pounds per square inch), the stresses due 
to this sudden loading being those actually computed from the load with 
the impact. For other proportions of impact, less than 100 per cent, 
the allowable working stress may be increased in a ratio equal to the 
percentage of impact. For dense structural yellow pine, the allowable 
working stresses would, therefore, be as follows: 

1. For wet or submerged loca¬ 


tions . .. 1,100 + 1,100 I pounds per square inch. 

2. For exposed locations 

(bridges). 1,400 -f- 1,400 I pounds per square inch. 


3. For constantly dry locations 1,600 + 1,600 I pounds per square inch 

where 1 is the proportional impact 
stress. 

17. Decay of Timber. —Decay of timber is caused by certain 
fungi or bacteria which gain entrance, develop, and finally destroy 
the fiber, reducing it to powder, if dry, or to soft pulp, if wet. 

The germs gain entrance sometimes from water, sometimes 
from the air, and their entrance is facilitated by checks. For the 
development of the germs, certain conditions of temperature and 




Table IV.— Working Unit-stresses for Structural Timber 
Expressed in Pounds per Square Inch. [Am. Ry. Eng’g. Assoc. Manual, 1915] 


STRUCTURAL ENGINEERING 


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WOOD 


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34 


STRUCTURAL ENGINEERING 


moisture are necessary. Dry wood will not decay, simply 
because it is dry, nor will wood that is completely submerged all 
the time or completely saturated. Extremes of wetness or 
dryness both prevent rot. 

“Wet-rot” was erroneously supposed to be due to moisture, 
and “dry-rot” to dryness, but both are due to the development of 
organisms of decay, which require different degrees of moisture 
and temperature. 

Sapwood decays more readily than heartwood, as the sap con¬ 
tains compounds that favor decay, while the resins of heartwood 
resist it. Wood does not decay because it is old. If the organ¬ 
isms do not gain entrance and develop, it may last forever. 

Wood, if unprotected and exposed to the weather, may decay 
in a few years by wet-rot, the alternate soaking with water, and 
then drying out, favoring decay. Wooden bridges are therefore 
generally covered by a roof, and exposed wood is painted. 
Decayed wood can be discovered by its softness, and also to some 
extent by its color, though there are discolorations that are 
not caused by decay. It lacks resonance when struck with a 
hammer, absorbs much water, and sometimes has an odor that 
indicates decay. The writer, in inspecting wooden railroad 
structures for many years, carried a cane or rod with a sharp 
point at the end. Often a wooden stringer would be found with 
a large part of the corner (sapwood) soft and useless. It is 
better to remove such rotten wood, for it helps the remaining 
sound wood to decay. Wherever moisture can collect and be held 
for quite a time, then drying out, rot is likely to occur. Sills 
and caps of pile trestles, and connections of wooden structures, if 
exposed, are likely to go first; and piles in earth decay at and near 
the ground level. Piles in water decay first near the water level. 

On the other hand, in dry places, a timber may look perfectly 
sound, yet may be a mere shell, the inside decayed to powder. 
Such timbers will sound hollow when struck with a hammer. 
The writer found a bridge truss of white pine in which, at one 
end, three out of four sticks in the chord were so decayed. A 
long slender auger may be used for boring into suspected timbers. 

Timber is sometimes attacked by animals which destroy it. 
White ants, which attack some timbers in the southern states, 
may eat away the entire interior, leaving only a shell. 

18. Marine Borers. —Timber in sea water is subject to attack 
by the teredo, the limnoria, and the chelura. The teredo enters 


WOOD 


35 


the wood, turns at right angles, and bores a tube from 34 inch to 
1)4 inch in diameter, which it lines with a calcareous lining, and 
in which it lives. Sometimes a pile will be completely perforated 
by these tubes, and in warm waters, such as the Gulf of Mexico, 
a 6 inch pile may be destroyed in two or three months. The 
teredo works as far north as the Gulf of St. Lawrence, though of 
smaller size there than in warmer waters. 



Fig. 9. —Results of limnoria and teredo action on piling. (Trans. Am. Soc. C. E., 
1922, p. 1413.) 


The limnoria and chelura are crustaceans, which attack wood 
on the outside for a limited distance above and below low-water 
level. 

There are also some forms of fresh-water wood borers. 

Figures 9, 10, and 10a show the work of the limnoria and 
teredo. These are taken from the discussion by W. G. Atwood, 
in the Trans. A.S.C.E. for Aug., 1922, pages 1413 and 1421. 




STRUCTURAL ENGINEERING 


30 



Fig. 10.—Limnoria attack on piles on Atlantic seaboard. (Trans. Am.jSoc. C. E., 

1922, p. 1413.) 



Fig. 10a.—Douglas fir piling after four years’ service. (Trans. Am. Soc. C. E. 

1922, p. 1421.) 






WOOD 


37 


An excellent discussion of this subject, indeed, the best that 
the writer knows, is contained in the three reports of the San 
Francisco Bay Marine Piling Committee for 1921, 1922, and 
1923. 

19. Preservation of Timber. —There are several methods used 
to protect wood against decay; (1) Seasoning; (2) an outside 
coating; (3) interior preservatives. 

Seasoning tends to preservation because it dries out the sap. 
It may, however, cause checking and so reduce the strength and 
provide a lodgment for germs, which all come from the outside. 
It is beneficial as far as decay is concerned, but seasoned woods 
that are to be exposed to the weather should be protected by 
external coatings or by internal antiseptics. 

Outside protection by paint is generally practiced and excludes 
germs. A poisonous coating like creosote is often effective. 
Piles are sometimes protected by being covered with nails with 
large heads, by copper, or other metal, by cement or concrete, or 
other coatings. These may be effective as long as they remain, 
but they may be destroyed by blows or abrasion. Charring the 
outside is often effective. This method is used on fence posts. 

Inside preservatives are applied either by soaking the timber, 
or by putting it in a vacuum to extract as much sap and moisture 
as possible and then soaking in the preservative, or by forcing the 
preservative in by pressure. Many preservatives have been 
tried, but the two now most used are creosote and zinc chloride, 
though corrosive sublimate (kyanizing) is sometimes used. 
Creosote is the most effective, and is the only one that is effective 
against wood borers. Creosoted piles have withstood the attacks 
of the teredo for as long as 40 years. 1 The great advantages of 
creosote are that it is not soluble and therefore will not wash out, 
that it is poisonous to germs and wood borers, and that it does not 
injure the wood. Its disadvantages are its odor, its black oily 
appearance, and the increased inflammability. Zinc chloride is 
the cheapest preservative, but is easily washed out and injures 
the timber. In dry locations it may be effective for many years. 
The use of zinc chloride is known as Burnettizing. There are 
standard specifications for these preservatives and for their use. 

The quantity of creosote injected should depend on the use. 
Railroad ties sometimes have as little as G to 8 pounds per cubic 
foot injected, piles from 12 to 24 pounds per cubic foot. 

1 Snow. 


38 


STRUCTURAL ENGINEERING 


20. Slow-burning Wood. —Woods that have been treated with 
certain chemicals are sometimes termed “fire-proofed wood”; 
but wood cannot be made so that it will not burn. Salts which 
have water of crystallization, which is given off under heat, and 
some which give off non-inflammable gases, that retard combus¬ 
tion, may be injected into it but neither of these are of much 
efficiency. Others, on decomposing, leave behind a non-volatile 
fluid residue which covers the wood with a thin glaze which keeps 
out the air; of these, the most efficient is phosphate of ammonium. 

There are paints which are not themselves inflammable, but 
they soon blister. Fireproof doors in factories are covered with 
tin, which protects the wood. 

21. References. —With reference to decay, preservation, and 
wood borers, see Snow, who has especially good chapters on these 
subjects, with voluminous references. 

Also Hoxie: “Dry Rot in Factory Timbers,” published by 
the Associated Factory Mutual Fire Insurance Cos., 184 High 
St., Boston. 

Handy Book on Painting, published by John T. Lewis & Bros. 
Co., Lafayette Building, Philadelphia, Pa. 


CHAPTER III 


THE CONSTITUTION, HEAT TREATMENT, AND MECHAN¬ 
ICAL TREATMENT OF IRON AND STEEL 

1. The constitution of iron or steel is different from its 
composition. The latter term refers to the proportions of the 
different elements, while the former refers to the condition in 
which those elements or their combinations are found. Two 
steels may have the same composition, but very different consti¬ 
tutions and physical characteristics, depending upon the heat 
treatment or the mechanical treatment to which they have 
been exposed. The constitution is often called the “ proximate 
composition;” the composition, the “ultimate composition.” 
The latter is the field of the chemist; the former is that of the 
metallographer. As Sauveur says “The analytical chemist may 
tell us, for instance, that a steel which he has analyzed contains 
0.50 per cent of carbon, without our being able to form any 
idea as to its properties, for such steel may have a tenacity of 
some 75,000 lb. per square inch or of some 200,000 lbs., a duc¬ 
tility represented by an elongation of some 25 per cent or prac¬ 
tically no ductility at all; it may be so hard that it cannot be 
filed or so soft as to be easily machined, etc.” The structural 
engineer should be informed regarding these matters, the changes 
which take place in the solidification and cooling of iron and 
steel from a molten state, the effect of heat treatment and 
mechanical work, and the substances which are produced, as 
shown by the “constitutional” or “equilibrium” diagram. 
These matters are fully explained in the books mentioned in 
Chap. I, particularly clearly and concisely in those of Stough¬ 
ton and Mills, and more in detail in those of Rosenhain and 
Upton. The following fundamentals should be familiar to the 
engineer. 

2. In a liquid solution of one substance in another, the two, 
while not chemically combined, are so intimately mixed that 
they virtually form one liquid in which no indication of hetero¬ 
geneity can be discovered by any physical instrument, even a 

39 


40 


STRUCTURAL ENGINEERING 


microscope or a polariscope. The solution may be in varying 
proportions, up to a maximum, or saturated solution, and the 
solution has properties which partake of the properties of the 
constituents. A chemical compound, on the other hand, con¬ 
tains the constituents in single definite proportions, it is a single 
definite substance, and it may, and generally does, have proper¬ 
ties entirely different from those of either constituent. 

If a liquid solution freezes, the constituents may or may not 
separate. If it freezes as a single substance, it is a solid solution , 
though not a chemical combination; and the constituents may 
change, as by precipitation, or change of crystalline form, even 
while solid, under variations of temperature or pressure. A 
liquid solution cannot freeze as a solid solution unless the 
constituents can crystallize alike. A metal always solidifies by 
forming crystals. If the different constituents crystallize in 
different forms, each would form its own crystals in freezing, and 
they would separate. 

In a solid solution, as in a liquid solution, the dissolved 
substance may exist, in general, in various proportions up to a 
maximum or saturation point. This saturation point may be 
different in a solid solution from that of the liquid solution 
from which it is formed. If the saturated solid solution 
contains less of the dissolved substance than the liquid solution 
from which it is formed, the excess will separate out when 
solidification occurs. 

3. Molten iron or steel is a liquid solution, mainly of carbon in 
iron. Pure iron, the element, does not occur apart from other 
elements, though crystals of nearly pure iron (ferrite) occur in all 
commercial irons and steels. The most important impurity is 
carbon, though there are others, such as silicon, phosphorus, 
sulphur, and manganese, which have important effects upon the 
properties of the resultant metal. For the present, and in 
the “constitutional” diagram, only the effect of carbon is 
considered. 

Carbon dissolves in molten iron up to a maximum of about 7 
per cent, though rarely over 4.5 or 5 per cent occurs. It forms a 
solid solution with iron up to about 1.7 per cent. Solid iron or 
steel may therefore have up to this proportion of carbon in solid 
solution, but no more; if there is more carbon in the metal, it 
is not in solid solution, but is separated out, in either one of two 
forms, namely, (1) cementite (Fe 3 C) which is a chemical com- 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 41 


pound (not a solid solution) containing 6.67 per cent carbon , 1 or 
( 2 ) graphite, which is nearly pure amorphous carbon. 

4. In the diagram Fig. 11 , ordinates are temperatures centi¬ 
grade, and abscissas are percentage of carbon. It shows the 
changes that occur as the liquid solution cools. Above the lines 
AE and EC the metal is a true liquid solution, though authorities 
are divided as to whether it is a solution of carbon in iron or of a 
carbide of iron in iron. AEC is the line where metal of any com¬ 
position will begin to solidify as it cools, and is called the liquidus, 



or lower boundary of the region in which the metal is completely 
liquid. If the percentage of carbon is less than about 1.7, the 
metal will freeze as a solid solution of carbon in pure iron (or of 
iron carbide in iron). Thus, if there is 1 per cent C it will freeze 
as a*solid solution of the same proportions. When it solidifies, 
the iron is in one of its three allotropic forms (see Art. 5), and 
is called 7 -iron. The solid solution is called austenite, which 
may therefore have any proportion of C up to about 1.7 per cent, 
at which point it is saturated with carbon. 

If the percentage of C is over 1.7 and less than about 4.3, there 
is too much C for the metal to freeze as a solid solution; hence 


,_ 12 _ =1 

3 X 56 + 12 15 


6.67 per cent. 







































42 


STRUCTURAL ENGINEERING 


saturated austenite will separate when the temperature falls to 
the lin eAE, and the remaining solution will be enriched in carbon. 
The more C in the liquid, the lower its freezing temperature, and 
as the temperature falls and more and more saturated austenite 
separates, the composition of the remaining liquid follows down 
the line AE. If, for example, the original solution had 3 per cent 
C , it would begin to solidify at about 1,250°, at point T; and as 
the temperature falls below this, saturated austenite separates. 
When the temperature has fallen to 1,200°, the liquid will 
have the composition indicated by the point S, or about 3.6 
per cent C. 

If there is over 4.3 per cent C, the first substance to separate as 
a solid is either graphite or cementite, according to circum¬ 
stances, and this precipitation leaves the liquid with less C than 
before; and, as the temperature falls, its composition follows 
down the line CE, while graphite or cementite solidifies. 1 

The point E represents the composition of the liquid when it 
must solidify as a whole, forming an alloy (not a solid solution) 
of saturated austenite and either cementite or graphite. The 
solution at this point is called the eutectic , and represents the com¬ 
position having the lowest melting point. A solution having the 
precise eutectic composition (4.3 per cent C) would not solidify in 
cooling until the temperature fell to 1,135°, and would then im¬ 
mediately form saturated austenite and either graphite or cement¬ 
ite (or both). Below 4.3 per cent C, cast iron is hypo-eutectic; 
above 4.3 per cent C it is hyper-eutectic . 

The line EC is slightly different according as graphite or cement¬ 
ite is precipitated from a solution having over about 4.3 per cent 
C. Slow cooling favors the formation of graphite, rapid cooling 
that of cementite. Similarly, the cementite which forms when 
the eutectic freezes may be later separated into ferrite and 
graphite, according to the rate of cooling and the other impurities 
present. Depending on the relative effects of the various factors, 
the metal may be a white cast iron, with almost all the carbon 
combined in cementite, or a gray cast iron, with the carbon in 
the form of graphite. 

The substances in the different areas of the diagram are there¬ 
fore the following: 

1 . . .or, cementite may first separate in every case, and at the high 
temperature existing may be decomposed into graphite and ferrite. 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 43 


I: A liquid solution of carbon or carbide of iron in iron; 

II: A mixture of solid austenite (C in 7 -iron) in a liquid solu¬ 
tion. The austenite is saturated (1.7 per cent C ) at DD'. Freez¬ 
ing begins on AE and ends on AD. 

Ill: A mixture of solid saturated austenite in a liquid solu¬ 
tion. Freezing ends at DE where the metal has the eutectic 
composition. 

IV: A mixture of solid cementite or graphite in a liquid solu¬ 
tion. Freezing ends on EF, where the metal has the eutectic 
composition. 

The eutectic freezes on DF. The line ADEF is called the 
“solidus,” and is the upper limit of the region in which the metal 
is completely solid. 

V: A solid solution, austenite; 

VI: A mixture of larger crystals of saturated austenite which 
crystallized in passing through III, and the eutectic, which solidi¬ 
fies as an intimate mixture of smaller crystals of saturated austen¬ 
ite with cementite or graphite. As the solid cools through VI, 
the austenite passes through changes still to be explained when we 
study the cooling of austenite below AD; it becomes ferrite and 
cementite. 

VII: A mixture of the cementite or graphite which solidified as 
the solution cooled through IV, with the solidified eutectic. 
A metal with exactly the eutectic composition would consist of 
cementite, graphite, or both/with saturated austenite. 

The structure just below AD is a homogeneous solid solution, 
while below DF it is heterogeneous. 

5. Pure iron (ferrite) exists in three allotropic forms: a-iron, 
which is magnetic, and which is the form below the line G'GHK; 
/ 3 -iron, which is non-magnetic, and occurs in area VIII; and 
7 -iron, which is non-magnetic, and which is the form in 
austenite. 

Consider now the cooling of solid steel below the line AD. 
At AD it is white hot austenite, and as this solid solution cools it 
changes its constitution in a manner similar to that in which a 
liquid solution changes its constitution in passing through area 
II. If there is less than about 0.5 per cent C, the cooling metal 
will meet the line BG; when it does, the austenite is immediately 
decomposed, even though solid, ferrite in the / 3 -form separates, 
and therefore the remaining constituent becomes enriched in 
carbon, and follows down the line BG in composition. Thus, a 


44 


STRUCTURAL ENGINEERING 


metal with 0.2 per cent C at 800°, 1 would consist of /?-ferrite to¬ 
gether with austenite corresponding to the point N, that is, with 
about 0.35 per cent C. At the line G'G, the /3-ferrite changes to 
a-ferrite and becomes magnetic, 2 while with further cooling the 
austenite follows down the line GH. Thus a-iron occurs up to a 
temperature of about 760° provided there is less than 0.5 per 
cent (7, and if there is over 0.5 per cent C up to a lower tem¬ 
perature, varying with the percentage of carbon, and about 690° 
if the carbon is 0.85 per cent. /3-iron only occurs if there is 
less than 0.5 per cent C, and at temperatures from 760° to a point 
varying with the percentage of C from 760° at 0.5 per cent to 
about 900° when there is no carbon. 

If there is more than about 0.85 per cent carbon, 3 the cooling- 
metal will meet the line HD, and where it does the austenite will 
be decomposed, cementite will be separated, the remaining 
austenite will be poorer in carbon C than before, and will follow 
down the line DH. Thus H is a point similar to E, except that 
it is in the solid. The composition at H is that of a solid eutectic, 
or eutectoid, as it is called to distinguish it from E. 

Thus a steel with 0.2 per cent C in cooling from the liquid state 
would remain as austenite down to about 840° when /3-iron would 
separate; at about 760° the /3-iron changes to a-iron, and at 
about 700° there is a-iron and the eutectoid. A steel with 1.4 
per cent C would remain as austenite down to about 980°, when 
cementite would separate, and at 700° there would be cementite 
and the eutectoid. 

The eutectoid is called pearlite, and consists of a very finely 
divided mixture of minute crystals of a-ferrite and cementite, 
into which the austenite separates at about 700°. 

The remaining areas of the diagram therefore consist of: 

VIII: /3-ferrite and austenite; 

IX: a-ferrite and austenite; 

X: “free cementite” and austenite; 

XI: previously formed a-ferrite, and pearlite; 

1 All temperatures are centigrade. 

2 Some metallurgists believe that the line GG' does not mark an allo- 
tropic change, and that j8-iron does not exist. They think that GG' is 
merely the lower limit at which all the iron has been changed from y to the 
a-form. 

3 Some books give the eutectoid composition as 0.9 per cent C. 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 45 


XII: previously formed cementite (“free cementite ”)> and 
pearlite; 



Fig. 12.—Micrograph of Armco ingot iron, or ferrite, magnified 125 times. 



Fig. 13.—Micrograph of eutectoid steel (0.85 per cent C) showing finely laminated 
pearlite; magnified 300 times. 

XIII and XIV: previously formed cementite or graphite, and 
pearlite. 






46 


STRUCTURAL ENGINEERING 


In Fig. 12 is shown a micrograph of Armco Ingot iron, almost 
pure ferrite. Figure 13 shows eutectoid steel, or finely lami- 


Fig. 14.—Hypo-eutectoid steel (0.35 per cent C); the dark is pearlite, the light 
ferrite; magnified 100 times. 




Fig. 15. —Hyper-eutectoid steel; the dark is pearlite, the light is cementite 

nated pearlite; Fig. 14 shows hypo-eutectoid steel (0.35 per cent 
C), showing pearlite and ferrite; Fig. 15 is hyper-eutectoid steel. 








CONSTITUTION AND TREATMENT OF IRON AND STEEL 47 


For all the micrographs in this book the writer is indebted to 
his colleague, Professor A. Sauveur. 

6 . Alloys with less than 1.7 per cent C are classed as steels; 
those with over 2.2 per cent C are cast irons; those with from 1.7 
to 2.2 per cent are intermediate, and are not represented by com¬ 
mercial products. Graphite is not found in steels, except possi¬ 
bly in a very high-carbon steel verging on cast iron. Pure iron 
in the 7 -form never occurs: it is always in solid solution with 
carbon as austenite. But if we could start with pure iron and 
heat it, it would successively be a-iron, /3-iron, and 7 -iro/i. 

A steel with carbon below about 0.2 per cent is called a 
low-carbon steel; with from 0.2 to 0.5 a medium steel; with from 
0.5 to 0.6 hard steel, and above 0.6 a high-carbon steel; though 
these limits are not standardized. 

Sauveur ( loc . cit. 1912, p. iv, 1) says that the following terms 
are those most commonly used: 

Very low carbon steen, very mild or extra 

mild steel, very soft or dead soft steel... Carbon not over 0.10 per cent 

Low carbon steel, mild steel, soft steel. Carbon not over 0.25 per cent 

Medium high carbon steel, half hard steel. Carbon 0.26 to 0.60 per cent 

High carbon steel, hard steel. Carbon over 0.60 per cent 

Very high carbon steel, very hard or extra 

hard steel. Carbon over 1.25 per cent 

7. It has been assumed thus far that carbon was the only 
impurity. There are others in all irons and steels, and in the 
“alloy steels” some constituents are added in considerable 
amounts to produce certain qualities. These change the various 
points of the diagram; silicon and phosphorus, for instance, having 
the effect of moving points D and E to the left. The lines of the 
diagram are determined by experiment, and some which have 
been drawn straight should, if accurate, be curved. There are 
other modifications which have not been shown. 

Considering only binary alloys, or those of two elements 
only, it is evident that in cooling they may act in different 
ways if one dissolves in the other when liquid. There are two 
extreme cases: 

(a) The two can crystallize in the same form, and in solidifying 
they crystallize together, each crystal being composed of the 
two elements just as the liquid solution, no matter what the 
proportions of the two. This is a solid solution. 





48 


STRUCTURAL ENGINEERING 


(b) The two crystallize differently, and separate entirely in 
solidifying, the state of solution existing when liquid being 
entirely destroyed, and the solid being composed of separate 
crystals of the single constituents. 

There are intermediate conditions, in which there may be a 
solid solution up to a certain proportion of one element in the 
other, and a separation when that proportion is exceeded, as in 
the case of iron and carbon, as above explained. 

Solutions of more than two elements give rise to more 
complicated conditions, which are explained in the detailed works 
on the subject. 

As a liquid alloy solidifies, the parts which freeze first push 
away the still liquid parts, and the former therefore contain an 
excess of the primary metal, while the parts which solidify last 
contain a larger proportion of the eutectic and other relatively 
fusible constituents. 

8 . We have seen that the solid solution, austenite, in cooling 
below the line AD, Fig. 11, undergoes molecular changes when 
the lines BGH, G'G, LH, and HD are crossed. These changes 
are accompanied by the evolution of heat, or retardation of the 
rate of cooling, shown by an increase in the time necessary to cool 
a given number of degrees; this occurs when the austenite is 
decomposed on the line BG with separation of /3-ferrite, also on 
G'G when the /3-ferrite changes to a-ferrite; and a still greater 
evolution of heat, with glowing of the metal, known as 
recalescence, when the eutectoid becomes pearlite on LH. These 
change-points, beginning with the one at the lowest temperature, 
are known as A h A 2 and A 3 . The points of corresponding 
change when heating are not at precisely the same temperatures 
as the points when cooling, owing to a lag or hysteresis which 
retards both transformations. The lag depends upon the 
rapidity of heating or cooling, being smaller the slower the tem¬ 
perature change, and upon other circumstances; so that the 
points of change on heating are called Aci, Ac 2 , Ac 3 , and those on 
cooling Ar x , Ar 2 , Ar 3 ; c standing for chauffage, or heating, and r 
for refroidissement, or cooling. If the composition is to the right 
of G there are only two points, namely, where LH is crossed 
(Aci or Ari) and where GH or HD are crossed, and the latter is 
known as Ac 2 _ 3 or Ar 2 _ 3 . If the composition is precisely the 
eutectoid, with about 0.85 per cent carbon, there is but one point, 
at H, known as Ac t _ 2 -3 or The higher the carbon, in LH, 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 49 


the nearer the points 2 and 3, or the points 1 and 2-3. The 
temperature between the two corresponding points for heating 
and cooling, between Ac 3 and Ar 3 for example, may be from 
25° to 50° C. 

Temperatures from B (about 895°) down to L (about 690°) 
for steel having less than 0.85 per cent carbon (known as hypo- 
eutectoid steel) and from D (about 1,135°) down to H (690°) 
for steel with above 0.85 per cent carbon (known as hyper- 
eutectoid steel), constitute the critical or transformation range. 
Above this range thesteel is a homogeneous solid solution, 
austenite; while below LHK it is called by Howe “a conglom¬ 
erate or granitic mass, a mixture of pearlite with either fer¬ 
rite or cementite according to whether it contains less or more 
than 0.85 per cent of carbon.’’ For any given steel, the critical 
temperature is on the line BGHD, though, for reasons that will 
be referred to, HD is not often used, and the critical temperature 
is often considered to be on BGHK. 

The properties of steel are extremely variable, depending not 
only upon the amount of carbon and other elements in it, but 
particularly upon the heat treatment and the mechanical treat¬ 
ment it has received. The engineer should be familiar with the 
facts and principles involved. 

Ferrite, by itself, is soft and ductile; cementite is very hard 
and brittle, and it is not desirable to have much of it in steels for 
structures or machinery, where exposed to shocks or blows, or 
where a failure would be serious or involve loss of life. “Free” 
cementite, or that formed above the line DH is undesirable; that 
present should preferably be in pearlite, that is, very finely 
divided and mixed with ferrite, and not in large crystals. Even 
in tool steel, which is exposed to blows, the cementite should 
if possible be as pearlite. Below 0.85 per cent C there is more 
pearlite as the carbon increases; above 0.85 per cent C there is 
less pearlite as the carbon increases. 

All the carbon in the steel is in the cementite, which contains 
1 Hso = Ks of its weight of carbon. The percentage of cemen¬ 
tite in steel is therefore 15 times the percentage of carbon. Steel 
of the eutectoid composition contains about 0.85 X 15 = 12.75 
per cent of cementite and 87.25 per cent of ferrite. Almost all 
steels for structures or machines have less than 0.85 per cent C; 
those with more, or to the right of H in Fig. 11 are tool steels and 
some spring steels, 


50 


STRUCTURAL ENGINEERING 


If a hypo-eutectoid steel is heated from the conglomerate or 
granitic condition in which it exists at ordinary temperatures, at 
the line LH, or point Ac h the pearlite is immediately trans¬ 
formed into austenite. With further heating, the austenite 
progressively absorbs the ferrite with which it was originally 
mixed. (To the right of II it absorbs the cementite.) When 
the a-iron changes to /?- or 7 -iron it loses its magnetism, and when 
it reaches area V it is all austenite solid solution, the carbon or 
carbide being entirely dissolved in 7 -ferrite. Below Ac 3 there 
are ferrite islets diffused in the austenite; these ferrite grains 
become coarser as the heating goes on, particularly in low-carbon 
steels, and this grain growth is accelerated by previous over¬ 
strain by cold working; but at Ac 3 they are shattered and become 
very fine. 

For reasons that will be explained presently, it is desirable 
that the grain of steel be as fine as possible. This will be accom¬ 
plished if it is heated to just above the line BGHK. If that line 
is barely crossed, particularly in hypo-eutectoid steels, the grain 
will be the finest possible owing to the breaking up of the ferrite 
crystals at Ac 3 . It must be kept at that heat only long enough to 
allow the complete diffusion of the last of the ferrite in the 
austenite. To heat higher would hasten this diffusion, but would 
coarsen the austenite grains by contact and coalescence. Long 
and high heating above Ac 3 , followed by slow cooling without 
mechanical work would coarsen the grain and hinder the trans¬ 
formations in subsequent cooling, tending to preserve the 
coarsened grain of the austenite, which will be coarser the higher 
the heat and the smaller the percentage of carbon. Coarse 
grain is accompanied by reduction of tensile strength, the more 
as the carbon is less; and by reduction of ductility, the more as 
the carbon is greater. The finest grain will be in steel of the 
eutectoid composition, heated to just above II; because above 
LII the new austenite grains grow, and yet the old grain size 
of the ferrite cannot be reduced till Ac 3 is reached. It is 
the same with steels above 0.85 per cent carbon; the old grain 
size of the cementite cannot be reduced till IID is crossed, 
but this line is so steep that if heated to HD the austenite is coars¬ 
ened, so that it is better to heat to just above HK, particularly 
since the excess or “free” cementite is small. 

9. If steel which has been heated above the critical tempera¬ 
ture is allowed to cool slowly without mechanical work, the grains 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 51 

grow, more slowly as the temperature falls, depending upon the 
composition. If it has been heated far above Ac 3 it will be very 
coarse. If it has been heated nearly or quite to the melting point, 
it will be burned, and will contain oxides or gases: such steel is 
generally ruined, and* can only be made good by remelting. If 
very low-carbon steel (under 0.15 per cent C ) is kept for several 
days at a temperature from 500° to 750°, or below Aci, the crystals 
become very large, provided it has been cold-worked, and the 
strength and ductility are greatly reduced: this is known as 
“ Stead’s brittleness.” 

10. Hardening. —The molecular transformations described 
require time. If sufficient time is not allowed, that is, if the tem¬ 
perature changes too rapidly, there is the lag above referred to, 
and the changes take place at a higher temperature in heating and 
at a lower temperature when cooling. Further, the change of 
austenite into pearlite is not abrupt, but there are three inter¬ 
mediate stages recognized, namely, martensite , troostite, and 
sorbite. Martensite is the hardest, strongest, and least ductile, 
and the others are progressively less hard, less strong and more 
ductile, till pearlite is reached, which is the softest, weakest, and 
most ductile. (Cementite, however, a component of all steels, is 
harder than martensite.) If the cooling is sufficiently slow, the 
transformation is complete at about 700°, and the steel becomes 
pearlitic. If the cooling is very rapid, as by quenching in cold 
water, the transformation temperature may be lowered to about 
300°, and the transformation is arrested at the stage of marten¬ 
site. With intermediate ratfes of cooling, the temperature of 
transformation will not be reduced so much, and troostite or 
sorbite may be produced; and the properties of the steel will 
vary accordingly. In this way steel is hardened by sudden cool¬ 
ing from the critical temperature Ac 3 or above. Hardening is 
produced by any element or treatment which throws the cooled 
metal into the form of martensite or an intermediate product 
between austenite and a mixture of either ferrite and pearlite 
or cementite and pearlite. 

We have seen that the finest grain will be produced if the steel 
is heated to just above the critical temperature and kept there 
just long enough for thorough diffusion of the ferrite (below 0.85 
per cent C). If the steel is suddenly cooled from this tempera¬ 
ture, the fineness of grain will persist, and the steel will become 
fine-grained and hard; martensite, troostite, or sorbite being 


52 


STRUCTURAL ENGINEERING 


produced according to the rapidity of cooling. If cooled from 
a higher temperature, the hardness will be no greater, but the 
grain-size will be larger. If heated ever so little below Ac 3 and 
cooled, it will not be hardened. It should therefore be heated 
to just above Ac 3 to produce the finest .grain combined with 
hardness. The heating should be slow at the end, in order to 
bring all the metal to the desired temperature, and not heat the 
exterior far above it. In this hardened condition, however, it is 
so brittle as to be unsuitable for most uses; and, moreover, serious 
internal cooling stresses are produced, which must be removed. 

If the steel is above 0.85 per cent C, the best temperature for 
hardening is just above the line HK, because that gives the best 
grain structure. If heated higher there would be more danger of 
cracking when quenched. 

Hardening depends upon and increases with the percentage of 
carbon. Pure iron is hardened practically none at all by sudden 
cooling, and the same is true of the free ferrite which separates 
in steel below 0.85 per cent C and of the cementite which separates 
in steel above 0.85 per cent C. It is only the pearlite or eutec- 
toid upon which hardening depends. Hardening therefore only 
affects the ferrite and cementite which forms the eutectoid, which 
on slow cooling becomes pearlite, but which on rapid cooling 
takes one of the intermediate forms. The 7 -iron of austenite is 
harder than /3- or a-iron, and on rapid cooling some of it remains in 
the 7 form. When it exists in this form at ordinary tempera¬ 
tures, in martensite, troostite, or sorbite, due to sudden or rapid 
cooling, it is not so stable as when produced by slow cooling, as 
it can be in some of the alloy steels; and these intermediate 
forms may therefore, when produced by sudden cooling of carbon 
steel, be more completely transformed toward pearlite by mode¬ 
rate heating. 

All the carbon present in steel forms the carbide Fe 3 C, which 
12 X 100 

contains 3 ^" 55 Ap f 2 = P er cen ^ ^ so > s ^ nce 12 parts 

of carbon form 180 parts of Fe 3 C or cementite, one part of carbon 
will form 15 parts of cementite. Hence the percentage of cemen¬ 
tite is 15 times the percentage of carbon. Pearlite contains 
0.85 per cent carbon, hence it contains 15 X 0.85 = 12.75 per 
cent cementite, and therefore 100 — 12.75 = 87.25 per cent 
ferrite, or approximately 6.8 parts of ferrite by weight for each 
part of cementite. Again, one part of cementite in pearlite will 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 53 



Fig. 16.—Hardened hyper-eutectoid steel; the light is martensite, the dark 

troostite. 



Fig. 17.—Hardened hyper-eutectoid steel; the light is martensite, the dark 

troostite. 






54 


STRUCTURAL ENGINEERING 


100 


produce = 7.8 parts of pearlite; and one part of ferrite in 


100 


cementite will produce = P ar ^ s pearlite. 


In hypo-eutectoid steel, all the carbon present, and hence all 
the cementite, is in the pearlite, hence hypo-euteetoid steel must 
contain a 

percentage of pearlite = C X 15 X 7.8 = 117(7 
in which C = percentage of carbon. 



Fig. 18.-—Sorbite in steel. 


The balance is free ferrite. 

In hyper-eutectoid steel, all the ferrite present is in the pearlite, 
and since 

percentage of cementite = 15(7 
percentage of ferrite = 100 — 15(7 
it follows that 

percentage of pearlite = (100 — 15(7)1.15 = 115 — 17.25(7 

The balance is free cementite. 

Figure 16 shows hardened hyper-eutectoid steel, with marten¬ 
site and troostite. Figure 17 shows a similar steel with smaller 
magnification, which brings out better the structure of the 
martensite. 

Figure 18 shows a micrograph of sorbite; and Fig. 19 is marten¬ 
site in hardened steel, magnified 800 diameters. 




CONSTITUTION AND TREATMENT OF IRON AND STEEL 55 


In order to produce homogeneity, it is desirable that the eutec- 
toid and the free ferrite should be thoroughly intermingled; hence 
the desirability of heating to Ac 3 , when the ferrite is thoroughly 
diffused in the austenite, and, when suddenly cooled, does not 
have time to separate and coalesce. This is one reason why a 
fine grain is desirable. 

It is impossible to cool carbon steel fast enough to produce 
much austenite, though a little may be produced by sudden 
cooling of high-carbon steels; the most rapid cooling practicable 
produces martensite. In alloy steels, however, austenite may be 



Fig. 19.—Martensite in hardened steel. Magnified 800 diameters. 

produced. When pure iron is quenched, the ferrite crystals 
have merely to rearrange themselves in changing from the y 
to the /3 and a form, without changing their positions; but, as the 
carbon content increases, the austenite, in cooling, expels the 
freed ferrite into the inter-crystalline spaces, causing both it and 
the carbide to travel, and this requires time; hence the trans¬ 
formation is hindered if the cooling is sudden, and intermediate 
products are the result. Thus pure iron is not hardened by 
sudden cooling, and the effect is greater as the carbon increases. 
If quenched from above HD, martensite is produced, but it will 
be coarse; if the composition is that of the eutectoid, and the 
metal is quenched from just above H, the structure will be very 




56 


STRUCTURAL ENGINEERING 


fine, and the product has been called hardenite, which is satur¬ 
ated martensite, or martensite with the eutectoid composition. 

11. Annealing and Tempering. —Annealing is a softening treat¬ 
ment, consisting in heating a hardened steel to slightly above the 
critical temperature Ac 3 , or to a lower temperature, and cooling 
slowly. If heated to just above Ac 3 , the softest and most ductile 
material will be produced, the internal stresses most effectively 
removed, and the steel will be pearlitic; but the tensile strength 
and yield point will be lowered. The time for which the steel 
is held at the annealing temperature is important, a longer time at 
a lower temperature producing the same effect as a shorter time 
at a higher temperature. The time depends upon the thickness. 

To give greater strength and yield point than would result 
from heating to Ac 3} while still retaining sufficient ductility with¬ 
out brittleness, the steel should be made sorbitic. This is done 
by heating to just above the critical point, thus refining the grain; 
then producing martensite by sudden cooling while preserving 
the fine grain, because in sudden cooling the ferrite or cementite 
grains do not have time to grow or coalesce as they may in slow 
cooling. Such hardened steel, however, is too hard and brittle 
for most uses, so that it is reheated to a temperature below 700°, 
kept there a suitable time, and then either slowly cooled or 
quenched, or quenched after cooling to a proper point to produce 
sorbite or whatever product is desired. This treatment is called 
tempering or drawing. Gradual separation of the finely divided 
pearlite, and coalescence of the cementite, which is undesirable, 
would be produced by long heating at 700°. In the second heat¬ 
ing to some temperature below 700°, the metal is so rigid that 
coalescence of grains does not occur to a harmful extent, and by 
proper treatment sorbite, troostite, or an intermediate structure 
may be produced. This is therefore a strengthening treatment , 
which preserves the fineness of grain. 

If great hardness is desired, and brittleness is not objection¬ 
able, the steel may be made martensitic by sudden cooling, or as 
much of the hardness as is objectionable may be removed by a 
slight tempering. 

It would be possible to produce sorbite by a single cooling, 
properly regulated, or quenched at the desired point, but the 
grain would not then be as fine as by the tempering process. 

Tempering is sometimes called “drawing/’ or “drawing the 
temper.” In doing this, the desired temperature is estimated 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 57 

by the so-called “temper colors” shown by the steel when heated, 
caused by the formation of a thin film of oxide on the surface. 
These colors are: 

Pale yellow. 220° C. = 428° F. Brown-purple. . . 265° C. = 509° F. 

Straw. 230° C. = 446° F. Purple. 280° C. = 536° F. 

Light brown. 243° C. = 469° F. Bright blue. 288° C. = 550° F. 

Brown. 255° C. = 491° F. Pale blue. 300° C. = 572° F. 

Dark blue. 315° C. = 599° F. 

The higher the temperature of tempering, and the longer the 
steel is kept at that temperature, the softer will be the metal. 

Steel which has been hardened by quenching is not much 
affected by subsequent heating to 100° C.; the particles are not 
mobile enough: but steel which has been stressed above the 
yield point, so that the elastic limit has been reduced to nearly 
zero, owing to the plasticity of the intercrystalline amorphous 
cement, rubbed off by the slipping, will have its elasticity entirely 
restored by a period of rest, which gives the amorphous material 
time to harden, or as Muir has shown, 1 by a short exposure to a 
temperature of 100° C. 

By “heat treatment” is therefore to be understood hardening, 
tempering, annealing, normalizing, and similar operations. 

Normalizing is annealing by heating to above the critical tem¬ 
perature for a certain time, and cooling in air, which is said to 
give a fine-grained, pearlitic structure. Normalizing, however, 
does not always leave the steel in the same condition. Much 
depends upon the size of the piece. A small piece would cool 
more rapidly than a large piece, and would become sorbitic, while 
a large piece would become pearlitic, or would have a different 
structure on the outside and on the inside. The strength and 
other properties are therefore, even in the same kind of steel, not 
necessarily made uniform by normalizing, though they may be 
made more uniform than they previously were. 

Sudden cooling from above the critical range always produces 
internal stresses, especially in thick pieces, and is liable to cause 
cracks, especially if there were previous initial stresses. Hence 
a softening treatment is often given before hardening. A piece 
may be so thick that it cannot be hardened throughout, but 
only on the surface. A temperature A 3 is too high for quenching 
any but small pieces. Cracks produced by quenching are often 
intercrystalline, as the cooling takes place suddenly when the 

1 Phil. Trans. Royal Soc., 1899. 










58 


STRUCTURAL ENGINEERING 


amorphous material is plastic. Cooling should only be as rapid 
as is consistent with avoiding undue cooling stresses; it should 
be slower the greater the least thickness of the piece. 

If a low-carbon steel already has a fine grain, produced by 
mechanical working (see Art. 12), it may be made soft, and the 
internal stresses removed, by suitable heating at a temperature 
below the critical range, say at 600° C. Important castings, 
especially if thick, should be annealed to remove cooling stresses; 
and structural steel which has been heated subsequent to rolling, 
such as eye-bar heads, or which has been stressed by cold working 
above the elastic limit, should be annealed. 

Iron in the 7 -form and in the /3-form is non-magnetic; only in 
the a-form is it magnetic. The proper heat for annealing may be 
found by means of this property. For steel between G and H in 
composition the critical temperature is reached, in heating, when 
it becomes non-magnetic, since the iron is in the a-form below, and 
in the 7 -form above that line. If a magnet is suspended outside 
the heating furnace, the proper heat is reached when the iron no 
longer attracts the magnet. For steel between G and B in com¬ 
position, since /3-iron is non-magnetic, the steel becomes non¬ 
magnetic above GG', while the critical temperature is on BG; 
the difference can be estimated in heating. 

12. Effect of Mechanical Work. —If steel is heated well 
above the critical point Ac 3 , and allowed to cool slowly, we have 
seen that the grain will be coarse. This is only true, however, if 
no mechanical work is put upon it; if it is rolled or hammered, the 
crystals are broken down, by crushing or shearing, into smaller 
ones, and the final size of grain is dependent upon the temperature 
at the finish of the mechanical working. The greater the amount of 
mechanical work the better, and the working should be finished 
about at the line BGHD. There is here a directional effect, 
however; the more working there is in a given direction, as by 
rolling, the more the ductility in a transverse direction is de¬ 
creased. Each passage through the rolls reduces the grain- 
size, and in the interval between passes it grows again. Work 
done on the metal below that line increases the brittleness, while 
if the work is finished at a higher temperature the strength will be 
less. Heavy working should stop somewhat above the line 
BGHD, moderately heavy working may be continued to lower 
temperatures, and light working still lower; the proper tempera¬ 
tures depending on the carbon content, and also on the percentage 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 59 


of other impurities. As working below that line increases brittle¬ 
ness, great care should be taken that the steel is not worked too 
cold. It is very dangerous to do even light working on steel at 
temperatures between 150 and 550° C., especially between about 
250 and 350°, which is the region of so-called blue heat. The 
temperature at which colors appear, however, depends somewhat 
on rapidity of heating. 

The above shows the necessity of having a proper proportion 
between the size of the ingot and the size of the finished piece. 
If the ingot is too small, the finishing temperature may be too 
high; if the ingot is too large, the finishing temperature may be 
too low, unless the piece is reheated. 

We thus see that if steel is heated considerably above the criti¬ 
cal temperature Ac* the grain may be made fine either by mechan¬ 
ical working or by heat treatment. It should never be heated 
above Ac 3 without being refined in one of these ways. 

13. Initial Stresses Due to Mechanical Work. —If steel is 
worked hot, initial stresses are not produced if the finishing 
temperature is high enough, except that they may be produced 
by unequal cooling; the working does not produce them. If it is 
worked cold, or below a temperature at which the crystals are 
mobile enough to adjust themselves, initial stresses are produced 
by the working, which are greater the lower the temperature 
at which the metal has been worked. Nevertheless, it is desirable 
to continue the working to below the temperature at which the 
grain coarsens by further cooling, for the coarseness is worse than 
the initial stresses and the so-called “ strain-hardening.” 

It is generally stated that hot working does not harden the 
metal, while cold working, which involves stressing above the 
elastic limit, does harden, producing strain-hardness . This 
depends upon what is meant by hardness, and the different 
meanings of that word have been referred to in Chap. IV of the 
“Strength of Materials.” Cold working, or stressing above the 
yield point, does of course harden in the sense that it raises the 
elastic limit and yield point, and sometimes the ultimate tenacity, 
diminishes the ductility or total stretch before fracture, and also 
reduces the ultimate resilience or capacity to resist shock; and 
increases the Brinell hardness. It makes the stress-strain diagram 
assume a shape like the line d 2 dub in Fig. 59 “Strength of Mate¬ 
rials,” the origin being at d 2 . Cold-worked metal has therefore, 
no doubt, less resistance to shock, greater brittleness (or less total 


(30 


STRUCTURAL ENGINEERING 


stretch), higher initial stresses, and probably, if the overstress has 
been severe enough, less resistance to repeated or alternating 
stresses. 

Cold working may be by wire-drawing, rolling, or hammering. 

Cold-rolling .—Let AC and HG, Fig. 20, be the surfaces gripped 
by the rolls as the piece is pushed through toward the right. 



Fig. 20. 1 —The backward flow of metal during reduction is greater along 
the axis than on the surface of the bar and approximately in the direction shown 
by the arrows. 


Disregarding the slight swelling at K and L, just back of the rolls, 
what has become of the original volume represented by ABC and 
HDG ? According to Howe and Groesbeck (A.S.T.M., 1920, 
Pt. II), the friction of the rolls prevents backward flow of the 
metal along the surfaces, but the inward pressure causes the metal 
in the central part of the piece to flow backward or in the direc- 



Fig. 21. 1 —Strip entering the initial 
pass. 



tion of the horizontal arrow, producing initial longitudinal 
compression at the center, to balance which there must be initial 
longitudinal tension at the surface. To test this, two strips of 
equal thickness, Fig. 21, were put through the rolls together; they 
curved apart as shown in Fig. 22, convex to each other. This 

J “A Note on Stresses Caused by Cold Rolling,” by H. M. Howe and E. C. Groesbeck. 

(Am. Soc.for Testing Materials, Vol. XX, Part II, pp. 32 and 36, 1920.) 













CONSTITUTION AND TREATMENT OF IRON AND STEEL 61 

shows that in the solid piece the central part would be in longi¬ 
tudinal compression (since it would stretch longitudinally if 
split in the middle) and the surface in longitudinal tension, or 
else that there is transverse tension on a plane like PN, or both. 
It is difficult to conceive that transverse compression on the 
upper and lower surfaces can leave tension on PN (see below). 
R. W. Woodward etched away the upper half of a rolled piece, 
and the lower half curved like the lower plate in Fig. 22. We 
may therefore accept it as proved that there is a longitudinal 
compression at the center and tension at the rolled surfaces, on a 
plane perpendicular to the direction of rolling. 

On a longitudinal plane, parallel to the direction of rolling and 
perpendicular to the piece, such as OEDB in Fig. 20, there should 
be little or no initial stresses if lateral expansion in rolling is 
prevented, as in rails or shapes, or plates rolled in a universal 
mill, which rolls all four edges. If lateral expansion occurs, the 
central parts are forced out laterally, and are left in compression, 
while the rolled surfaces are in tension. 

On a longitudinal plane such as PN in Fig. 20, there is no reason 
why, with uniform rolling, the internal stress should vary in 
different parts, still less that it should be tension at some points 
and compression at others, as would have to be the case for 
equilibrium; hence there is no initial stress on such a plane, 
though it looks, at first sight, as though these would be the very 
planes on which there would be. 

In wire drawing the wire is pulled through the die instead of 
being gripped between rolls. Here there may be backward 
surface flow rather than inward flow, leaving initial compression 
near the surface and tension near the center. Moreover, the 
surface is stressed higher than the inside, and more strain- 
hardened and strengthened. Sometimes wires draw hollow, 
or with breaks or cavities along the axis; or fail first at the center 
under a tension test. 

Hammering is local, and tends to cause central flow in all 
directions, leaving compression at the center and tension at the 
surface. 

It is often stated in discussions on cold working that, 
in drawing, tension remains at the surface and compression 
inside, while rolling and forging leave compression at the surface 
or just the opposite of the explanation above given. 

Hot working does not produce these effects, but if the working 


62 STRUCTURAL ENGINEERING 

is continued to too low a temperature, the effect may be that of 
cold working. 

The material close to sheared edges or punched holes has had 
the elastic limit raised by overstrain, and is often termed brittle, 
having less shock resistance and less ductility. 

It is generally better to avoid the use of metal which has been 
severely cold-worked, in places where it may be subjected to 
shock or to alternating stresses, or else to remove the effects of 
the cold working by annealing. 

On the other hand, it is desirable to use metal which has been 
properly hot-worked, and finished at a suitable temperature. 
Insufficient working means superficial working, that is, a fine¬ 
grained exterior and a coarse-grained interior. The proper 
finishing temperature may be ensured by providing a limit of 
shrinkage from the last pass through the rolls down to normal 
temperature. Thus the specifications of the A.R.E.A. and of 
the A.S.T.M. provide, in the case of steel rails, as follows: 

The number of passes and speed of the train shall be so regulated 
that on leaving the rolls at the final pass the temperature of the rail 
will not exceed that which requires a shrinkage allowance at the hot 
saws, for a rail 33 feet in length, and of 100 pounds section, of 6% inches, 
and inch less for each 10 pound decrease in section (or inch more 
for each 10 pound increase in section). 

It appears, however, that the above shrinkage allowance is 
too great, and allows finishing some 270° C. above the critical 
range. In Technologic Paper 38 of the Bureau of Standards, it is 
even stated that this shrinkage clause “has no significance 
whatever.” 1 However, stopping mechanical work at a temper¬ 
ature above the critical point will not give as large grain as merely 
heating to that point and slowly cooling; the effect of mechanical 
work persists. The manufacturer naturally likes to finish at as 
high a temperature as possible, as the rolling is easier. 

There is also a difference between Bessemer and open-hearth 
steel as regards shrinkage, the above-named paper giving 
for Bessemer, coefficient of expansion = 0.0000146 per degree 
Centigrade; for open hearth, coefficient of expansion = 0.0000156 
per degree Centigrade. There is a small contraction, in heating, 
at about 700°. 

1 See also Trans. Am. Inst. Mining Eng., 1914, on “ Finishing Temperatures 
and Properties of Rails.” 


CONSTITUTION AND TREATMENT OF IRON AND STEEL 63 

14. The So-called Cold Crystallization of Steel. —Steel some¬ 
times breaks in service—as a member of a bridge, a car axle, 
or other piece—and the break shows a crystalline fracture. It 
has often been claimed that this has been due to vibration or to 
repetition of stress, which has crystallized the metal. It is be¬ 
lieved by the best authorities now that there is no such thing as 
cold crystallization. The crystalline appearance is due either to 
the manner of fracture, or to improper heat treatment. A piece 
of steel may be broken by a sudden blow, and show a crystalline 
fracture, because the parts did not have time to draw down and 
the break is across the crystals, without elongation; the same steel 
broken in a testing machine would show the usual ductility and 
reduction of area, with a silky fracture. It may also show a 
crystalline fracture, with no drawing-down, if such drawing-down 
is prevented by the fact that the piece was a notched specimen 
(see Chap. IV). As a matter of fact, since steel is composed of 
crystals, all fractures show crystals under the microscope; by 
crystalline fracture is therefore properly meant a coarsely crystal¬ 
line structure. This can only be due to the heat treatment, 
or because the break is by fatigue (see Chap. XXII of the previous 
volume). There is no proof that vibration has caused, or can 
cause, large grain size in steel. If such grain exists, it existed 
before the steel was put into service, if it has not been heated, 
though the fact will not be disclosed till a fracture occurs. 

15. Welding of Steel or Iron.—Parts of steel or iron may be 
welded together by being heated to a temperature near the melt¬ 
ing point, when the material is soft and pasty, and pressing and 
hammering the parts together. The iron oxide which forms at 
such high temperature is dissolved by a flux. The parts to be 
welded are pressed together in such a way as to squeeze the slag 
out. The grain will be very coarse, owing to the high heat, if 
allowed to cool without working, so that the metal must be 
hammered continuously till below the line BGHK. The danger in 
welding is that some parts of the pieces, not at the weld, but a 
little away from it, will be heated to the area V without receiving 
the necessary work while cooling. A properly made weld will 
not break at the weld, but will often or generally break near the 
weld at a stress much less than the original strength, sometimes 
not over half. The defect to be feared is therefore coarse crystal¬ 
lization near the weld. To make a welded bar of proper strength, 
it should be reheated to the line BGHK to restore the grain. The 


64 


STRUCTURAL ENGINEERING 


lower the carbon, the more easily steel will weld. It is generally 
considered that cast iron will not weld, and high carbon steel with 
difficulty; but by actually melting the adjoining parts, cast iron 
may be welded. 

EFFECT OF STRESS AND STRAIN 

16 . The engineer should know something of the effect of stress 
and strain on steel from the point of view of its crystalline 
composition. The following summary may assist him: 

When the crystals form in cooling, they grow from a starting 
point or nucleus by coalescence or building up, as frost grows on a 
window pane. When fully formed, there will be interstices 
between the crystals, which will not fit accurately together. 
These interstices are not voids, but are filled, according to the 
best working hypothesis, by an amorphous or colloidal material 
formed during the original cooling, which has not yet crystallized, 



A- Before Sfronnincj 



Fig. 23.—Diagram of the formation of slip bands. ( Rosenhain ). 

perhaps because it is held in equilibrium between conflicting 
tendencies to crystallize with adjacent crystals. This amorphous 
material is an inter-crystalline cement, and, when hard, is 
stronger than the crystals themselves, as shown by the fact that 
a fracture is not around the crystals following the boundary 
lines, but through the crystals themselves. The boundary lines 
are surfaces of strength, not of weakness. At high temperatures, 
however, near the critical point, the amorphous material 
becomes weak and plastic before the crystals do, and then a tensile 
break will occur around the crystals and not through them, 
and there are few or no slip bands (see below). 





CONSTITUTION AND TREATMENT OF IRON AND STEEL 65 


Under tension, the steel, composed of a mixture of crystals 
of ferrite and pearlite, that is, of ferrite and cementite, with 
intercrystalline amorphous cement, will elongate, the ductile 
ferrite crystals elongating most, but being restrained by the 
intermingled brittle cementite, which elongate little. If the 
elastic limit is not exceeded, they resume their original shapes 
when the stress is removed. When the stress becomes great 
enough, there is slipping on oblique planes through the crystals, 
which would form a series of steps on a polished surface, as in 
Fig. 23, which would show as a series of parallel lines, or slip 
bands, on the surface of any one crystal, as the planes of slip would 
be parallel in that crystal. The planes of first slip would be the 
weakest planes. These planes, however, would have different 
orientation in adjoining crystals, and would not form continuous 
planes through the entire piece, but would be interrupted at the 
crystal boundaries, and therefore would show under the micro¬ 
scope as parallel lines on each crystal, but in different directions 
on the different crystals. This shows another reason why a fine 
grain increases the strength and elastic limit, because the slip 
planes are more often interrupted and changed in direction than 
if the grain is coarse. Another reason is that the finely divided 
brittle cementite crystals support better the intermingled ferrite 
crystals. 

The slip bands, being steps, show dark under the micro¬ 
scope under normal illumination, because the light rays are 
dispersed by them and do not return into the microscope; 
but under oblique illumination, they may show white on a 
dark ground. 

Figure 24 shows slip bands in ferrite. Figure 25 shows 
Neumann bands, which are only produced when the fracture is 
sudden, as explained in Chap. IV of the “Strength of Materials.” 

When slipping occurs, there is an abrasion or rubbing off of 
some material on the slipping surface, which thus becomes an 
amorphous layer along that surface. As long as this is plastic it 
acts like a lubricant, and the slipping continues, but it hardens 
quickly and regains its elasticity after a period of rest or under 
a moderate heat, say that of boiling water, and when it hardens, 
that slipping surface is stronger than it was before; so that a 
repetition of the same stress will cause no more slipping on those 
same planes, but slip will first occur on other planes, which are 
stronger, because there was no slip on them under the first load. 


66 


STRUCTURAL ENGINEERING 


Thus there may be two non-parallel sets of parallel slip bands on 
the surface of the same crystal as seen in the microscope. 



Fig. 24.—Slip bands in ferrite. 


This theory explains fully the phenomena of the yield point, 
the immediate reduction of the elastic limit to zero by overstress, 



Fig. 25.—Neumann bands in ferrite. Magnified 125 diameters. 

if retested immediately, the restoration of elasticity by a period 
of rest or by moderate heat, and the raising of the elastic limit 







CONSTITUTION AND TREATMENT OF IRON AND STEEL 67 


and ultimate strength by a stress above the yield point; but it 
does not explain the fact that an overstrain in tension lowers the 
elastic limit in compression and vice versa. Ultimately, under 
severe stress, when slip has occurred on many planes, the material 
becomes a mass of small crystalline fragments in relatively thick 
layers of amorphous material, and the hardness is generally 
increased while the ductility is diminished (because much of the 
final deformation has already occurred). 

The character of a fracture will be different according as it is 
caused by a gradually applied load, by shock, or by repeated or 
alternating stress. 

If caused by a gradually applied tension it will occur across 
the crystals, often along slip planes at some part of the surface. 
Since the crystals may be very minute, a fracture may look like 
a square break, and yet may be largely along shear planes. The 
fracture will look fibrous, especially if largely by shear. 

A blow does not give time for slip, and the fracture is across 
the crystals on cleavage planes, which are not necessarily the same 
as the slip planes. The fracture may look crystalline. 

Both fractures are across the crystals, not around them. 

A fracture by repeated stress occurs by the formation of a 
minute crack along a slip plane, which gradually extends across 
the section. It is not easy to see how this crack could form unless 
at each repetition there was a slight increase of slip (deformation) 
along that plane. This lends probability to the maximum strain 
theory. The fracture often looks crystalline, because when the 
crack has progressed some distance the load on the remaining 
section becomes eccentric, and the break is sudden, as by a blow, 
and may then occur across cleavage surfaces. The method of 
development of a crack is another reason why a fine grain is 
desirable, especially in a piece which is to be subjected to alter¬ 
nating stresses or to shock; because a crack can be less easily 
propagated across finely divided crystals than across the slip 
surfaces of a smaller number of larger crystals. There is more of 
the strong amorphous material when the grain is fine, and a slip 
plane must change its direction at each crystal boundary, so 
that the yield point is increased by fineness of grain. Large 
crystals mean continuous slip or cleavage surfaces over larger areas. 

In some alloys there may be brittle films of a different constitu¬ 
ent surrounding the crystals, and the break may be through this 
constituent. 


68 


STRUCTURAL ENGINEERING 


Under a gradual pull, the pearlite is apt to break before the 
more ductile free ferrite, because of the brittle cementite in the 
pearlite. The fracture, however, passes through both ferrite 
and cementite, though the strength of the latter alone is several 
times that of the former. Under alternate stress the fracture 
is more apt to originate in the ferrite, because it is weaker and 
slip bands form in it first. 

Iron or steel broken slowly at ordinary temperatures show 
slip bands, but if broken slowly at a high temperature when the 
intercrystalline amorphous cement is viscous, the break will be 
around the crystals and there will be few or no slip bands. If 
broken suddenly at a high temperature, the viscous material has 
not time to flow, the fracture will be through the crystals, there 
will be slip bands, and the strength will be higher than if broken 
slowly. 

17. The attempt has been made to give in this chapter a brief 
statement of the fundamentals which the engineer should know. 
The subject is a large and complicated one, and it is hoped that it 
has been made clear that the treatment of steel is a delicate 
matter, which affects the physical properties profoundly, and 
which should be under the supervision of a trained metallurgist. 

It is hoped that the reader may be stimulated to continue his 
study of the subject. For this purpose he is advised to begin by 
reading Stoughton’s book, and to follow it with Rosenhain’s 
“Introduction to Physical Metallurgy.” He should then study 
one, or better, all three of the general books on materials (John¬ 
son, Mills, and Upton), and the paper by H. M. Howe, on “Life 
History of Network and Ferrite Grains in Carbon Steel,” in the 
Trans. A.S.T.M. for 1911, pp. 262-386. Finally, he should study 
the large works of Howe and Sauveur, namely, 

Howe, H. M: “The Metallography of Steel and Cast Iron,” 
McGraw-Hill Book Co., Inc., 1916; 

Sauveur, A: “The Metallography and Heat Treatment of 
Iron and Steel,” 1916. 


CHAPTER IV 


CAST IRON 

1. Pig iron is the first product in obtaining iron or steel from 
the ore, and is obtained from the blast furnace. When pig iron 
is remelted and cast into molds, sometimes after mixing with 
other pig irons, it is cast iron. 

The blast furnace, fully described in the books on materials, 
is charged with fuel (coke), ore, and flux (usually limestone) in 
alternate layers, and the iron oxide is reduced, trickles down to 
the hearth, and is drawn off at intervals. The flux unites with 
the ash of the fuel and the impurities or gangue of the ore, to 
form slag or cinder, which floats on the iron, and is drawn off 
separately, to be wasted or used in making cement. The charac¬ 
ter of the flux should therefore depend on that of the impurities. 
If these are acid, such as silica and alumina, as they usually are, 
the flux is basic, such as lime or magnesia, and the slag will con¬ 
sist mainly of silicates of lime or magnesia. If the impurities 
are basic, or high in lime or magnesia, the flux is acid. Most of 
the sulphur in the ore or fuel unites with the lime in the flux. 
If the impurities are both acid and basic in proper proportion, 
the ore is called self-fluxing. The character of the flux may be 
computed if the ore and fuel are known, and may be regulated 
so as to produce a desired result. The iron is cast into so-called 
pigs, or taken in ladle cars to the mixer, to be directly converted 
into steel or wrought iron or used for castings, without remelting, 
or more frequently remelted and mixed. 

2. Cast iron is a complex material, and very subject to 
variation due to the character of the impurities and the details 
of operation of the furnace. It is coarsely crystalline, may be 
very hard, has a lower melting point than any other form of iron, 
but lacks toughness, and is not malleable or ductile, and cannot be 
welded except by smelting. It contains generally 3 to 4 per 
cent carbon (though sometimes up to 5 per cent or more), with 
varying amounts of other impurities, such as silicon, phosphorus, 
sulphur, and manganese, which greatly affect its structure and 

69 


70 


STRUCTURAL ENGINEERING 


properties. A typical cast iron would contain about 94 per cent 
iron, 3.5 per cent carbon, and 2.5 per cent other elements. It is 
of two principal grades, depending upon the condition in which 
the carbon exists in it, namely, white cast iron and gray cast iron. 1 

3. In white cast iron (Fig. 26) the carbon is mainly or entirely 
in the form of cementite , that is, chemically combined with the 
iron (ferrite). As this cementite (Fe 3 C) contains, by weight, 

one fifteenth carbon ^ 56 + 12/* ^ ^°^ ows that with 3 or 4 



Fig. 26.— White cast iron: the dark is pearlite, the light cementite. 

per cent carbon, if all combined, the cementite will constitute 45 
or 60 per cent of the whole, by weight, the balance being 
ferrite and impurities. The properties of white cast iron will 
therefore be largely those of cementite; it will be hard, brittle, 
difficult to machine, and very resistant to abrasion. It has 
few uses, and none in structures or machinery, but is useful as the 
wearing surface of parts exposed to wear, as car wheels and brake 
shoes. It is produced by suddenly chilling the molten metal, as 
by casting it against an iron mold. The sudden cooling of the 
surface does not give the graphite time to separate as it does when 
1 The constitution and structure of iron and steel, and many important 
characteristics, cannot be understood without a good knowledge of the 
elements of metallography, a brief statement regarding which has been 
given in the preceding chapter, which it is hoped may lead to a study of 
the subject in detail. 





CAST IRON 


71 


the cooling is slow. Certain impurities, such as sulphur and 
phosphorus, favor the production of white cast iron, while silicon 
favors the separation of the carbon as graphite. The fracture of 
white cast iron is silvery white, with little or no graphite flakes 
visible, because they do not exist in this material. One of the 
main uses of white cast iron is for the production of malleable iron 
(see Chap. YI). 

4. In gray cast iron (Fig. 27) the carbon is mainly in the form 
of graphite, deposited as flakes. Since iron is 3.5 times as heavy 
as graphite, it follows that if there were 4 per cent of carbon, all 



F IQ . 27.—Gray cast iron; showing the black graphite; the ferrite as white 
borders around the graphite; the dark pearlite; and the phosphide eutectic 
(steadite). 

in the form of graphite, 1 0 , or about 12.5 per cent of the volume, 
and 34 of a plane surface, would be graphite. 1 Hence a fractured 
section would show the gray color of graphite, with perhaps little 
metallic luster unless the graphite particles were brushed off with 
a stiff brush. The actual percentage of graphite in gray cast 
iron, by weight, is from 2 to 4, the cementite being generally 
under 1.5 per cent. Graphite is very weak, and the particles 
easily split, hence gray cast iron is weak in tension or under shock. 

Mottled cast iron is intermediate between white and gray. 
It has no uses. 

i 96 + 3.5 X 4 = 110; 12.5 per cent = H; M = K- 




72 


STRUCTURAL ENGINEERING 


5. Uses of Cast Iron.—Only gray cast iron is used in structures. 
It is not often used in tension. Formerly it was much used for 
beams, but is not now considered reliable for such use, and is 
only used for minor parts which are mainly or entirely in compres¬ 
sion, as for bed plates, column bases, and columns in buildings. 

On account of its cheapness, fluidity, high compressive 
strength, and the ease with which it may be cast, it is still one of 
the most useful engineering materials. About one-fifth of the 
pig iron produced in the United States is remelted and used for 
cast iron. It is used for car wheels, brake shoes, fly wheels, 
stoves, gears, water pipes, valves, and many machinery parts. 

It appears from Bauschinger’s tests that cast iron resists high 
temperatures, such as may occur in a conflagration, better than 
wrought iron; for although cast iron has a lower melting point, 
the temperature in a conflagration is generally far below such 
point, and at such temperatures as are reached cast iron bends less 
easily than wrought iron or structural steel. Both materials, 
however, if used in buildings, should be protected by fireproofing 
if damage by fire is to be avoided. 

6. Composition and Constitution.—Gray cast iron contains 
generally above 2 per cent graphite and less than 1.5 per cent 
combined carbon, and may be considered as an impure steel 
mechanically mixed with graphite. Its other impurities are 
mainly Si, S, P, and Mn. Silicon and sulphur can be regulated in 
the blast and remelting furnaces. Phosphorus cannot be regu¬ 
lated, and all that is in the ore goes into the cast iron or pig. 
Manganese can be controlled to some extent. 

Silicon occurs in foundry pig up to 3.75 per cent. It facilitates 
the separation of carbon as graphite, and thus increases softness, 
soundness, fluidity, and decreases shrinkage, but above 3 per cent 
it may cause hardness by uniting with the iron to form a silicide. 
In cast iron to be used for making steel, silicon should not be over 
1 per cent, in order to limit the quantity of slag and the loss of 
iron in the steel furnace. It also tends to cause the austenite to 
decompose into ferrite and graphite instead of into ferrite and 
cementite down to the eutectoid (not eutectic) point (H in the 
equilibrium diagram, Fig. 11). It should be remembered that 
the austenite in cast iron acts the same as in steel as the 
temperature falls. 

Sulphur favors the retention of combined carbon more 
powerfully than silicon favors the opposite effect, hence it tends 


CAST IRON 


73 


to produce a hard, brittle iron. It also forms a sulphide of iron, 
FeS, which makes the iron very tender at a red heat 
(“red-short”), and thus tends to cause cracks in castings, due to 
shrinkage when prevented by the mold. Sulphur comes mainly 
from the fuel, though sometimes from the ore also. It can be 
counteracted by manganese, which has a greater affinity for sul¬ 
phur, forming a sulphide that is much less injurious than FeS, 
which will not be formed if there is sufficient manganese. Hence 
sulphur is often limited, in specifications for castings, to not over 
0.10 per cent and sometimes to not over 0.05 per cent, and it is 
required that the Mn must be not less than three times the S. 
For steel making, some of the S in the cast iron may be eliminated 
by one of the basic processes. 

Phosphorus in the ore is not eliminated in the blast furnace, 
and as it is very objectionable in steel, it must not exceed 0.05 
per cent in ore having 50 per cent iron or 0.10 per cent in cast 
iron used for making steel by the acid process, but may be higher 
if the basic process is used, which eliminates most of the 
phosphorus. By thermo-electric smelting, with a slag rich in 
lime, the phosphorus may also be removed. 

Phosphorus in itself tends to increase combined carbon in cast 
iron, but it also makes the melted iron more fluid, and so 
lengthens the time of solidification, thus indirectly favoring the 
formation of graphite. This indirect effect, when there is high 
silicon, makes phosphorus, on the whole, favor the formation of 
graphite; but high phosphorus and low silicon favor the produc¬ 
tion of white iron. It makes the iron brittle when cold. 

Manganese increases combined carbon, but not to the extent 
that sulphur does; and so, to the extent that it combines 
with sulphur, it lowers the combined carbon. There should be 
enough manganese to prevent the formation of FeS; any greater 
amount increases combined carbon. It thus counteracts the 
bad effects of sulphur, but it makes the casting hard and diffi¬ 
cult to machine. Graphite increases the workability, not only 
because it is soft, but because it acts as a lubricant for the 
cutting tool. 

It is thus seen that the impurities in cast iron are mainly of 
importance in their effect upon the form that the carbon assumes. 

7. Shrinkage.—Cast iron, as its name implies, is always in the 
form of castings made by pouring the melted metal into a mold. 
In cooling, the metal solidifies and shrinks, the shrinkage depend- 


74 


STRUCTURAL ENGINEERING 


ing largely upon the proportion of graphite. When the eutectic 
freezes, it breaks up into a solid solution of carbon in iron (or 
carbide of iron in iron) and graphite, that is, into saturated 
austenite and graphite; or into austenite and cementite, the latter 
later breaking up into graphite and ferrite. In the molten metal 
the carbon is in solution, and hence does not increase the volume; 
but when the carbon separates as graphite, as it does in gray iron, 
it is the same as introducing a new substance, and the volume is 
increased. There is therefore an initial expansion when the 
graphite separates, followed by a gradual contraction in cooling 
further. Any substance, therefore, which favors the formation 
of graphite (such as Si), and any substance which favors fluidity 
(such as Si and P), produces sharp castings with relatively little 
shrinkage, while any substance that favors the crystallization of 
the carbide as white iron has the opposite effect. With much 
graphite the shrinkage may be very small. The allowance for 
shrinkage of gray cast iron is about one-eighth of an inch per foot; 
white cast iron, and steel, shrink about double as much as gray 
iron. 

As shrinkage is less the slower the cooling, and as large castings 
cool more slowly than small ones, shrinkage decreases as the size 
of the casting increases. 

It is obvious that, in designing castings, certain principles 
must be observed, with the object of lessening the internal stresses 
produced in cooling, and thus increasing the strength of the 
casting. These rules are: 

(1) Avoid sharp corners, especially re-entrant angles, by round¬ 
ing the corners off with curves of considerable radius, even if these 
have to be machined down afterward. In cooling, the outside 
cools first, while the inside remains hot or molten, and therefore 
heat is conducted outward. The flow of heat to the re¬ 
entrant angle will obviously be more free than to the surface on 
either side of it, and the isothermal lines will be bent toward the 
angle instead of being parallel to the surface. The crystals grow 
at right angles to the surface, and slower along a line bisecting 
the re-entrant angle than perpendicular to the adjacent surfaces. 
There will thus be initial tension at the angle and inward from it. 
Projecting corners are not so bad. 

(2) Avoid sudden changes in thickness; and do not connect 
parts of different thicknesses, as a thin rim of a wheel to thick 
spokes, except by a gradual curve; for the cooling will be more 


CAST IRON 


75 


rapid the thinner the metal, and hence the thicker metal, in 
cooling and shrinking after the thinner metal has become cold, 
will cause large internal stresses and perhaps cracks. If sudden 
variations in thickness cannot be avoided, it may be necessary 
to provide special means for cooling the thicker sections rapidly, 
so that internal stresses may be avoided or reduced. This may 
be done by chilling the thicker sections, by metallic pieces in the 
mold, or by a water gate, which is a column of porous material in 
the mold, down which water may be poured. 

(3) Avoid shapes in which the ends of the casting cannot con¬ 
tract without crushing the mold, which tends to produce stresses 
in the intermediate parts. Arrange if possible so that all parts 
of the casting may shrink without restraint and as uniformly as 
possible. 

The subject of foundry practice and the principles involved in 
making castings of cast iron or steel should be understood by the 
structural engineer, and he is strongly recommended to read Chap. 
X of Stoughton’s “Metallurgy of Iron and Steely” where other 
references are given; and also the chapters on cast iron in John¬ 
son’s “Materials of Engineering,” and in Mills’ “Materials of 
Construction.” 

8. Defects in Cast Iron (see Chap. XIII of Stoughton).—The 
defects in cast iron may be due to improper methods or careless¬ 
ness in manufacture, or to excessive amounts of impurities; 
or there may be defects developed in the process of casting. The 
former may be guarded against by limiting the amount of 
impurities, and requiring care in manufacture. Defects of the 
latter.class are: (1) Cracks (checks) due to errors in designing, 
which do not allow properly for contraction, and are especially 
likely to occur if the shrinkage is large or the sulphur high, since 
this element not only favors shrinkage, but also makes the iron 
red-short, or likely to crack when at a red heat. They can gener¬ 
ally be detected by inspection or by the sound of the casting 
when struck with a hammer. Car wheels are periodically tested 
in this way. (2) Segregation, or concentration of the impurities 
at one place. This generally occurs at the point which freezes 
last, since each layer which solidifies rejects some of its impurities, 
which pass into the still liquid metal. Segregation is less if 
the sulphur and phosphorus are low, especially if the carbon is 
also low. (3) Blow holes may occur, especially if the mold is not 
properly vented so that the gases may escape. (4) Porous or 


70 


STRUCTURAL ENGINEERING 


spongy structure may occur if the solidification does not proceed 
uniformly. 

9. Grades of Pig Iron. —Pig iron is usually classified according 
to the purpose for which it is to be used, as follows: 

Bessemer pig; for making steel by the acid Bessemer 1 or acid 
open-hearth process; 

Basic pig; for making steel by the basic open-hearth process; 

Malleable pig; for making malleable cast iron (see Chap. VI); 

Foundry pig; for making gray iron castings; 

Forge pig; for making wrought iron. 

The chemical composition of these grades is usually kept within 
the following limits, according to Mills: 



Silicon, 
per cent 

Sulphur, 
per cent 

Phosphorus, 
per cent 

Bessemer pig. 

1.0 to 2.0 

not over 0.05 

not over 0.10 

Basic pig. 

under 1.00 

under 0.05 

not specified 

Malleable pig. 

i 0.75 to 2.00 

not over 0.05 

not over 0.20 

Foundry pig. 

1.50 to 3.00 

not over 0.05 

0.50 to 1.00 

Forge pig. 

under 1.50 

under 0.10 

under 1.00 


10. Strength of Cast Iron. —The tensile strength is small, and 
depends largely upon the proportions of graphite and cemcntite, 
being greatest, according to Howe, when the carbon in cementite 
is about 1.2 per cent and the graphite 2.8 per cent in a total of 4 
per cent carbon. The specifications of the A.S.T.M. for gray iron 
castings provide the following minimum ultimate tensile strength 
in pounds per square inch: 

For light castings, having any section < 3^ inch thick; 18,000 
For heavy castings, having no section < 2 inches thick; 24,000 
For medium castings, not included in the above; s 21,000 
The stress-strain diagram in tension is convex upward, the 
value of E continuously decreasing from the origin. The initial 
value is about 30,000,000 for hard iron, 24,000,000 for average 
and 14,000,000 for soft iron; but at 10,000 pounds per square 
inch these values are reduced by one-half or more. The reduc¬ 
tion of area is scarcely appreciable, and the elongation rarely over 
3 or 4 per cent. Figure 28 gives some typical diagrams, but the 
material is very variable. 

1 The basic Bessemer process is not used in the United States. 













CAST IRON 


77 


The compressive strength of cast iron is large, varying greatly 
with the quality. Short specimens have ultimate strength from 
about 35,000 pounds per square inch for soft gray iron to over 
100,000 for hard white iron. The diagram shows that E is more 
nearly constant, up to a certain point, than in tension, especially 
for hard iron, the initial value being about the same as for tension. 

In flexure, the actual extreme stress, even for small loads, is not 
the same as the modulus of rupture computed from the load by 



Fig. 28.—Stress-strain diagrams for cast iron. (Mills.) 

the usual formula, because E is not constant, or precisely the same 
in tension as in compression for all loads. The computed 
modulus of rupture generally varies from 1.5 to 2.25 times the 
tensile strength, in solid rectangular sections, averaging about 1.8; 
Lanza 1 gives an average of about 2: for other forms of section it is 
less. Tests of round bars with spans of 12, 18, and 24 inches, of 
gray cast iron, gave the computed modulus of rupture varying 
from 1.63 to 1.87 times the tensile strength. 2 These relations are 

1 See Lanza’s “Applied Mechanics,” p. 378. 

2 A.S.T.M. vol. 10, p. 299. 
































78 


STRUCTURAL ENGINEERING 


important, because it is often necessary for the structural engineer 
to compute cast iron in flexure, as, for instance, in column 
bases. Computed by the flexure formula, the ultimate modulus 
of rupture will vary from about 39,000 for light gray iron castings 
to over 50,000 for heavy castings. 

Specifications of the A.S.T.M. for gray iron castings prescribe a 
flexural test of a round bar, the so-called “ arbitration bar,” which 
is 134 inches diameter at one end and 1 inches at the other (to 
allow easy removal from the mold), and 15 inches long. Tested 
on a span of 12 inches, such a bar must sustain a minimum central 
load of 2,500, 2,900, and 3,300 pounds for light, medium, and 
heavy castings respectively, and must deflect at least 0.1 inch at 
the center; cast iron for locomotive cylinders must sustain at 
least 3,200 pounds with a deflection of 0.09 inch. These 
loads correspond to values of the modulus of rupture of about 
39,100, 45,400 and 51,600, respectively. 

11. Allowable Stresses. —Owing to its unreliability, variability, 
and brittleness, the factor of safety for cast iron should be larger 
than for wrought iron or steel. As cast iron is now generally 
prohibited for bridges, there are no specifications for allowable 
stresses for that material in such structures. For buildings, the 
specifications of the National Board of Fire Underwriters allow: 


Pounds per 
Square Inch 


Cast iron in compression, short pieces. 

Cast iron in shear. 

Cast iron in flexure, compression side. 

Cast iron in flexure, tension side. 

Cast iron in columns 9,000 — 40- (Maximum - = 60) 

Y Y 


16,000 

1.500 
16,000 

2.500 


The cast iron to which these specifications refer is described as 
follows: 

Cast iron shall be of good foundry mixture, producing a clean, tough, 
gray iron. Castings shall be free from serious blow-holes, cinder spots, 
and cold shuts. Transverse tests on cast iron shall be made upon the 
l>£-inch diameter “Arbitration Bar” of the American Society for Test¬ 
ing Materials. The bar to be supported on 12-inch centers, loaded at 
the middle, and in no case shall it test at less than 2,900 pounds. Tensile 
tests optional. 

Water pipes of cast iron are exposed to hoop tension. Thick¬ 
ness of such pipes is standardized for various diameters and heads. 






CAST IRON 


79 


Thus, the A.S.T.M. specifies for a head of 400 feet, or a pressure 
of 173 pounds per square inch for a 48-inch pipe, a thickness 
of 1.96 inches, which gives a tensile stress of about 2,100 pounds 
per square inch. In such a pipe the hoop tension may exceed 
that due to the static head, due to water hammer. The stress 
allowed gives a factor of safety of at least 9. 

12. Protection of Cast Iron. —Water pipes are protected by 
dipping them, after heating to 300° F., in a varnish made of hot 
coal tar pitch and oil, to give them a smooth surface and protect 
against corrosion. Structural cast iron should be painted or 
enclosed in fireproofing. 

13. Semi-steel is a product made by melting a mixture of 
from 20 to 50 per cent of steel, with pig iron. It is therefore a 
cast iron of low carbon content and high strength, and is some¬ 
times used for castings requiring extra strength. It is neither 
steel nor cast iron. 


CHAPTER V 


WROUGHT IRON 

1. Wrought iron, technically, is the product of the puddling 
furnace. It does not necessarily differ chemically from low- 
carbon steel except that it has mixed with it, in layers or threads, 
1 to 3 per cent of slag. The difference between it and low-carbon 
steel is in the process of manufacture, wrought iron being formed 
by the aggregation of pasty masses, while all steels are formed in 
a molten mass. 

In the puddling furnace, pig or cast iron and iron ore or scrap 
or mill scale are melted on the hearth of a furnace lined with 
iron ore or some form of iron oxide. The slag is basic, and the 
impurities of the cast iron are reduced and largely eliminated by 
the iron oxide of the slag and of the fettling or lining, and the 
oxygen of the furnace gases. The nearly pure iron solidifies in 
pasty masses, its melting point being higher than the temperature 
of the furnace. The pasty masses, dripping with slag, are 
removed by the puddler, and either squeezed or hammered, and 
then rolled into flat bars called “muck bars.” These bars are 
cut into short lengths, piled in layers, generally all in the same 
direction but sometimes with the bars alternating in direction, 
and the pile is heated and rolled into “merchant bar.” This 
operation is repeated once or twice. 

It is customary to speak of wrought iron as fibrous. It is, 
however, not fibrous by structure, but only on account of the 
layers or threads of slag running in the direction of the rolling, 
which in a sense justifies the term. Figure 29 is a micrograph of 
wrought iron, in longitudinal section, showing ferrite and slag. 

Wrought iron is practically pure ferrite, with a small amount of 
impurities, and with some slag mechanically mixed with it. 
There is some pearlite. The carbon is generally below 0.12 per 
cent. It is much lower in manganese than steel, this being 
perhaps the best method of distinguishing the two chemically, 
for it has not been recarburized as steel has. It has the 
advantage of being free from blow-holes. It generally contains 

80 


WROUGHT IRON 


81 


more phosphorus than steel, but this element appears to be less 
objectionable than in steel. It generally has lower ultimate 
tensile strength, elastic limit, elongation, and reduction of area 
than soft steel, and under compression a short prism is more 
liable to crack. It does not harden when cooled suddenly from 
a high heat. It can be welded easily. 



Fig. 29. —Wrought iron; longitudinal section, showing ferrite and slag. Magnified 
100 diameters. 


2. Use of Wrought Iron.— Wrought iron was formerly the 
principal material used for metal structures, cast iron having 
been used for certain parts such as compression pieces and 
occasionally for beams. Most metal bridges built more than 35 
years ago are of wrought iron, and many of these are still in use. 
If the practice of the structural engineer is extensive, he is sure 
to meet them. Since the perfecting and cheapening of the 
methods of making reliable steel of any desired grade, even the 
softest, steel has practically entirely superseded wrought-iron 
for structures. Its principal uses now are in cases where its 
superior welding qualities are useful, as for certain rods and bars, 
and for welded pipe and boiler tubes; and in cases where its 
(supposed) greater resistance to corrosion is valuable, as for wire, 
stay bolts, tanks and other purposes. 

3. Strength of Wrought Iron— The tensile strength is greatest 
in the direction of rolling unless the muck bars have been cross- 
piled, when it may be about equal in both directions. Generally 




82 


STRUCTURAL ENGINEERING 


the strength at right angles to the direction of rolling is about 
three-fourths that in the direction of rolling. 

The specifications of the A.S.T.M. prescribe the following 
requirements for (ri) staybolt, ( B ) engine bolt, and ( C) extra 
refined wrought-iron bars, made from all-pig-puddled iron, with¬ 
out any admixture of iron scrap or steel, and for plates of class 
A' — from puddle bars made wholly from pig iron and such 
scrap as emanates from rolling the plates, and class B' = from 
puddle bars made wholly from pig iron or from a mixture of pig 
iron and cast-iron scrap, together with wrought-iron scrap: 






Wrought-iron plates 


A 


c 

Class A' 

Class B' 





Width, 

6-24 

inches 

Width, 

24-90 

inches 

Width, 

6-24 

inches 

Width, 

24-90 

inches 

Tensile strength, T, 
pounds per square 
inch. 

48-52,000 


48-54,000 

49,000 

48,000 

48,000 

47,000 

Section ^ square 

inches. 

49-53,000 

47-53,000 

Section > 1^ square 
inches. 







Yield point, by drop of 
beam, pounds per 
square inches. 

0.6T 


26,000 

26,000 

26,000 

26,000 

Section <, \y± square 
inches. 

0.6T 

0.6T 

Section 1 — 4 square 

inches. 


0.55T 

0.55T 





Section>4 square inches 
Minimum elongation in 
8 inches, per cent.... 
Minium reduction area 
in 8 inches per cent.. 


0.5T 

0.5 T 





30 

48 

25 

40 

25 

37 

16 

12 

14 

10 


The value of E is generally from twenty-six to twenty-eight 
million pounds per square inch. 

The tensile strength T is greater the finer the grain, and the grain 
will be finer the greater the amount of mechanical work put upon 
the material and the lower the temperature at which that work 
is finished. For the greatest fineness and strength, the rolling 
should not cease till the temperature has fallen about to the line 
LH in Fig. 11; i.e., to about 700° C. There is therefore an 
increase of strength the greater the proportionate reduction of 
area from the pile to the finished bar; not, as is often stated, the 




























WROUGHT IRON 


83 


smaller the bar. Commander Beardslee found the following 
results: 

When the bar was 6.62 per cent of the pile in area, T was 56,543 

pounds per square inch 
When the bar was 11.63 per cent of the pile in area, T was 51,848 

pounds per square inch 
with intermediate values for intermediate percentages. The 
effect of reduction of area in rolling is more pronounced on the 
elastic limit than it is on the ultimate strength. Rods % inch 
in diameter may show a true elastic limit as high as 35,000, while 
for rods 2 inches or more in diameter it may be as low as 25,000. 
The yield point of wrought iron is apt to be farther above the 



Fi G . 30.—Stress-strain diagram for wrought iron (two different scales). 


true elastic limit than in steel, sometimes the difference being 
nearly 10,000 pounds per square inch. 

Figure 30 is a typical stress-strain diagram. 

Cold working and overstressing above the elastic limit raises the 
latter, and also the ultimate, especially if the piece is allowed to 

Heating to a high temperature favors coarse crystallization, 
especially if the piece is maintained a considerable time at that 
temperature. Slow cooling also favors coarse crystallization. 
Mechanical work during cooling breaks up the coarse crystals and 















84 


STRUCTURAL ENGINEERING 


produces fine grain. The melting temperature of wrought iron 
is very high, and for welding it must be heated moderately close 
to that temperature; hence when welding is done the part heated, 
not only at the weld, but all that is heated above the critical 
range, should be refined under the hammer to below the critical 
range. Often this is done at the weld, but a weak spot is left 
a short distance from it, where the metal has not been worked. 

The compressive strength of wrought iron in short lengths is 
from about 45,000 to 60,000 pounds per square inch. The 
strength of long columns, as in the case of steel, will be given by 
a formula in which the numerator should be but little above the 
yield point, or from 25,000 to 35,000 pounds per square inch. 
The ultimate is for this case a little above the yield point. 

The shearing strength of wrought iron is frequently assumed 
as 80 per cent of the tensile. It is less on a plane parallel to the 
rolling, or in the direction of th e fibers, than on a plane perpendicular 
thereto, unless the muck bars were cross-piled. The difference 
between the strength in the two directions is sometimes 100 per 
cent. Mills states the shearing strength parallel to rolling 20,000 
to 35,000 pounds, and on a transverse plane 30,000 to 45,000 
pounds. Longitudinal shearing should be avoided, or the muck 
bars should be cross-piled; in the latter case the strength may be 
equal in the two directions but less than in a transverse direction 
without cross-piling. 


CHAPTER VI 


MALLEABLE CAST IRON 

1. This is white cast iron which has been made stronger and 
somewhat malleable, without fusion, by packing it in 
some pulverized material and annealing it at a temperature ol 
about 700 to 900° C. for from 60 to 100 hours, or longer. It has 
the advantages of cast iron, in low fusibility and ease of casting 
into forms which could be forged of iron or steel with difficulty 
or not at all, and also the advantages of much greater strength, 
toughness, and ductility than is possessed by cast iron. 

The weakness of cast iron is due to the fact that white cast iron 
is hard and brittle, while in gray cast iron the graphite is not 
uniformly disseminated through the mass, but occurs in compara¬ 
tively large flakes which form points or planes of weakness. 
If there were less graphite and if it were uniformly disseminated, 
the metal would be stronger. In white cast iron the carbon is 
almost all in combination with the iron, and is uniformly dissem¬ 
inated, for each particle or molecule contains its proportion of 
carbon. 

In making malleable castings, therefore, they are first made of 
white iron, and then annealed, by which the carbon is changed 
from the combined form to an amorphous graphitic form, not in 
flakes, but uniformly and finely disseminated through the mass, 
in which condition it is called “ temper-carbon.” A cast iron 
low in carbon is desirable, it is poured at a high temperature so 
as to be fluid, and it is cooled rapidly, to produce white iron. 
The silicon is comparatively low, to facilitate the formation of 
combined carbon, yet it must be high enough to allow the sepa¬ 
ration of carbon as graphite during annealing. Figure 31 shows a 
micrograph. 

If the material in which the castings are packed during 
annealing is inert, such as sand or clay, the heat alone produces 
the change in the condition of the carbon. The cementite is 
broken up into ferrite and carbon. If the packing is cinder or 
slag, or iron ore, the metal is decarburized, some of the carbon 

85 


86 


STRUCTURAL ENGINEERING 


uniting with the oxygen of the packing; and generally there is 
some decarburization in any case unless the annealing is in 
vacuo. If the process is carried far enough, the iron may be con¬ 
verted almost into comparatively pure steel—ferrite with some 
temper carbon. Usually there is a thin skin of decarburized 
material, with a center of ferrite and temper-carbon, forming the 
black-heart malleable iron usual in this country. With longer 
heating, as in Europe, there is greater decarburization. The 
cooling after annealing is as slow as possible. 

Even if malleable iron contains as much carbon as gray iron, 
the carbon as temper-carbon is much less weakening than as 



Fig. 31.—Malleable cast iron, showing ferrite white, and temper-carbon black. 

flakes of graphite; while the iron, not being in the form of a 
comparatively coarse matrix of ferrite and pearlite, is much 
tougher and less brittle. 

2. Strength and Other Properties.—The A.S.T.M. speci¬ 
fications require for malleable castings a minimum tensile 
strength of not less than 45,000 pounds per square inch, and a 
minimum elongation in 2 inches of 7.5 per cent. The tensile 
strength is thus about double that of gray iron castings. The 
reduction of area may be as much as 12 per cent. 

The crushing strength is no greater, and generally less, than 
that of cast iron, since the material more nearly approaches 
the character of wrought iron. The modulus of rupture is greater 




MALLEABLE CAST IRON 


87 


than for cast iron, or from 65,000 to 90,000 pounds per square 
inch because the tensile strength has been so greatly increased. 
The toughness, or amount of energy absorbed up to tensile 
rupture, is very much greater than for cast iron, which may show 
12 to 20 inch pounds per cubic inch, while malleable iron may 
show 250 to 500. 

Malleable iron has thus considerable ductility and malleability. 
It may be flattened, or even bent double. 

3. Shrinkage.—When the castings are first made which are 
later to be converted into malleable iron, since these castings are 
of white cast iron, the shrinkage is considerable, as almost 
no graphite forms. When, by annealing, the carbon in the 
cementite or white iron is converted into finely divided temper- 
carbon, there is an expansion which is about the same as what 
would have occurred if the graphite had been precipitated 
originally. This property must be allowed for in designing 
malleable-iron castings. 

4. Uses.—Malleable iron has a large use for castings whose 
shape is such that they cannot well be forged, but which must be 
stronger and tougher than gray cast iron, and for pieces subject 
to tension. It is used for many parts of railway rolling stock, 
couplers, journal boxes, brake fittings, etc.; for pipe fittings, 
many of which must be threaded; for parts of agricultural imple¬ 
ments; for many machinery parts; for tools; for post caps, beam 
hangers, column bases, and other structural purposes. Cast steel 
could be used for most of these purposes, but is more expensive. 

5. Case-hardening.—Analogous, but opposite, to the process 
of making malleable iron, is the process of case-hardening, by 
which a low-carbon steel, or wrought iron, is packed in a carbon¬ 
aceous substance, such as coke, charcoal or other form, from 
which it absorbs carbon into the outer layers, which become 
case-hardened and form a coating of higher carbon steel. Such 
a steel, when quenched, will be hardened on the outside while the 
soft inside core will be little affected. The heating should be to 
above the point Ac\. 


CHAPTER VII 


STEEL 

1. Steel results from the purification of cast iron, by which the 
carbon is reduced to the desired proportion, and other impurities 
either removed or reduced to proper limits; or by the addition of 
carbon to wrought iron. Beginning with pure iron, which can 
only be obtained in the chemical laboratory, as the carbon is 
increased a graded series of products results, from low-carbon 
steel to the cast irons, the latter varying in physical properties 
not so much in proportion to the amount of carbon, as accord¬ 
ing to the form in which it occurs. The steels also, as has been 
seen, vary greatly according to the constitution as well as the 
composition. 

Formerly steel was intermediate between wrought iron and 
cast iron, and was made from wrought iron by adding carbon, by 
the cementation or by the crucible process. While these pro¬ 
cesses are still used for some kinds of steel, almost all steel is 
now made from cast iron by the Bessemer or by the open-hearth 
process, by which the carbon is first almost all burned out, and 
then enough is added to produce the grade of steel desired. 

Some grades of steel, therefore, have no more carbon than 
wrought iron, and, like it, will not harden when suddenly cooled 
from a high heat. Other kinds, containing some alloying element, 
are, like cast iron, not malleable, except at certain temperatures. 

Ordinary or carbon steel has but small amounts of other 
elements except carbon. Alloy steels are those to which more 
considerable quantities of other elements are added for the 
purpose of producing desired properties: they will be briefly 
referred to presently. 

The logical line of division between steel and cast iron would 
seem to be at the maximum percentage of carbon which can exist 
in solid solution with iron above the critical temperature. This 
limit is at 1.7 per cent carbon. Below this, the carbon (or car¬ 
bide) is in solid solution above the critical temperature and the 
material is homogeneous, while below the critical temperature the 
materials separate, while solid, into crystals of different charac¬ 
ters, as explained in previous pages. Above the limit of 1.7 
per cent C, the carbon (or carbide) crystallizes out in solidifying, 

88 


STEEL 


89 


and the metal is not homogeneous, but consists of crystals of 
carbon or carbide in a mass of different constitution. Ordinarily 
steel has less than 1.7 per cent C and cast iron has over 2.5 per 
cent C; hence carbon steel usually contains 98+ per cent of iron, 
1.7— per cent of carbon, and some impurities. 

2. Processes of Manufacture.—The distinction between steel 
and wrought iron is not in composition, but in method of manu¬ 
facture. Wrought iron is made by melting cast iron in the pud¬ 
dling furnace, exposing it to a flame which burns out the carbon, 
gathering the wrought iron into pasty masses and removing it, to 
have the slag squeezed out and to be rolled into bars. Steel is 
made, in the Bessemer and open-hearth processes, by burning out 
the carbon and casting the liquid steel in an ingot. 

In the cementation process, wrought-iron bars are packed in 
boxes, surrounded by carbonaceous material. Under the high 
heat of the furnace, the carbon penetrates into the bars at the 
rate of about % inch in 24 hours, the slag in the wrought iron 
causes the evolution of gas, and the bars, when removed, are 
covered with blisters, so that this steel is known as “blister steel.” 
It is then melted. 

In the crucible process, wrought iron and carbon (charcoal) are 
placed in crucibles and melted into steel. Other elements may 
be added. In England it is customary to remelt blister steel. 

These last two processes are in general only used for making 
tool steel. 

The Bessemer and the open-hearth process may be acid or 
basic. These processes have for their object to burn out the car¬ 
bon and to eliminate the other impurities so far as possible, or to 
reduce them to proper limits. 

In the acid Bessemer process, air is blown through the molten 
cast iron, which is run into the converter from the mixers. The 
oxygen of the blast oxidizes the silicon, manganese., carbon, and 
some of the iron. An acid slag forms on top of the iron, and the 
lining of the converter must be acid, in order that the acid slag 
may not attack it and quickly eat it away. Sulphur and phos¬ 
phorus maybe oxidized, but, if they are, the acid slag will not take 
them up since they are acid themselves, and so they will be 
deoxidized and remain in the steel. It is therefore essential, in 
this process, to use a cast iron with no more of these elements 
than is allowable. When the carbon is nearly all burned out, a 
recarburizer, ferromanganese, is added, as it is cheaper to burn 
out almost all the carbon, silica and manganese, and then add the 


90 


STRUCTURAL ENGINEERING 


necessary amount than to try to stop the burning at the proper 
point, which would be difficult. 

In the basic Bessemer process lime is added to the molten 
metal, in order to form a basic slag, and the lining must be made 
basic in order that it may not be attacked. The basic slag will 
take up some of the sulphur and much of the phosphorus, which 
may therefore be removed by this process. 

The difference between the acid and basic processes is there¬ 
fore that in the former the slag is acid (that is, with excess of 
Si0 2 ), the lining is acid (silicious), and sulphur and phosphorus 
are not eliminated; while in the basic process lime is added in 
sufficient quantity to more than satisfy the silica, the lining is 
basic (dolomite or magnesite and tar), the sulphur and phos¬ 
phorus are taken up by the basic slag, so that a cast iron may be 
used with more of these elements than in the acid process. 
The lining plays no part in the process of conversion. 

In the Bessemer process the charge is comparatively small 
(10 to 15 tons), and the time of a blow short (about 10 minutes). 
The air blown through the metal leaves some dissolved gases, and 
some of the iron is oxidized, so that it is necessary to add a proper 
amount of recarburizer containing not only the desired amount 
of carbon, but also manganese and silicon, to deoxidize the iron, 
unite with what sulphur is left to form MnS instead of FeS, and 
remove the dissolved gases. 

In the open-hearth process, the oxidizing flame, produced by 
burning gas with air, sweeps over the surface of the molten metal 
on the hearth of the furnace instead of being blown through it. 
The charge is relatively large (50 to 200 tons), the time of a heat 
long (about 10 hours). The process may be acid or basic. 
Steel scrap, or iron ore, or both, are often added to the charge of 
cast iron. The process can be stopped at the proper point more 
easily than in the Bessemer process, yet it is customary to carry 
it further and to add a recarburizer, but a smaller amount than 
in the Bessemer process, since there are less dissolved gases and 
often the carbon has not been so completely burned out. 

In the basic process, whether Bessemer or open-hearth, the 
recarburizer cannot be added directly to the bath, for if that were 
done the basic slag would take silicon from the recarburizer and 
throw phosphorus back into the metal. The recarburizer must 
therefore be added after the slag has been drawn off, generally 
to the metal as it flows from the furnace, or in the ladle into which 
the finished metal is run. Even then, in the basic process some 


STEEL 


91 


slag may be carried over, and some phosphorus thrown back into 
the metal. 

In the Bessemer process no fuel is used, the impurities of the 
bath furnishing the fuel; in the open-hearth process the fuel is a 
pre-heated mixture of air and gas. In the Bessemer process 
none of the charge is melted in the converter, but is run into it 
melted from the mixers; in the open-hearth process some or all 
of the charge of pig iron, scrap, and ore is melted in the furnace. 

The basic Bessemer process is not used in the United States. 

3. Impurities.—The most injurious elements in steel are sulphur 
and phosphorus. 

Sulphur in the ore can be eliminated or reduced by roasting the 
ore. What remains, and what comes from the fuel, can be 
controlled in the blast furnace, so that there should be little in 
the pig. This can be partly eliminated in the basic steel process, 
and partly counteracted by adding manganese in the recarburizer. 
Sulphur in iron or steel exists as sulphide of iron or of manganese, 
and as manganese has greater attraction for it than iron has, 
manganese will take it from iron. 

The injury done by sulphur arises from the fact that manganese 
sulphide exists as small drops or masses which are liquid or pasty 
at the rolling temperature of steel, so that at this temperature 
the sulphur makes the steel tender and liable to crack, or “red- 
short.” Iron sulphide is more injurious than manganese sulphide 
because, instead of coalescing, it spreads out in sheets, and is 
more weakening. Manganese is therefore added to steel to take 
the sulphur from the iron, and the manganese sulphide normally 
goes off in the slag, though some may remain. Manganese also 
gets rid of the oxygen, by being oxidized and passing off with the 
slag in the acid process. In cast iron or cast steel the effect of 
sulphur is to make the castings liable to crack when solidifying, 
especially if the mold is such that shrinkage cannot occur without 
compressing the sand, so that it causes shrinkage tensile stresses. 

It is desirable, therefore, to have all the sulphur in the form of 
manganese sulphide, and there should be manganese enough to 
ensure this result, with some to spare. Since the percentage of 
Mn in MnS is 5 %2 times the percentage of sulphur, it is considered 
that in steel the percentage of manganese should be three to four 
times the percentage of sulphur. 

Phosphorus would do little harm if small in amount and 
uniformly disseminated, but it tends to become localized or 
segregated as phosphide of iron. 


92 


STRUCTURAL ENGINEERING 


Phosphorus is injurious because it makes steel brittle when 
cold, or “ cold-short,” especially if subjected to shock. This brittle¬ 
ness increases with the carbon. Phosphorus cannot be controlled 
in the blast furnace, but can be partly removed by the basic 
steel processes. As the basic Bessemer process is not used in the 
United States, it is essential that ore used here for making 
Bessemer steel should be low in phosphorus, the limit allowed 
being 0.05 per cent. Sulphur must also be low, as it is not 
removed in the acid Bessemer process, the usual limit in the 
cast iron used in this process being 0.2 to 0.7 per cent. In the 
basic Bessemer process, largely used in Europe, high phosphorus 
is desirable (from 1 to 3 per cent) as it forms an important part of 
the fuel. Ores of sufficient purity have hitherto been available 
in the United States, but are becoming scarcer. The basic 
process is less expensive than the acid process, because, although 
the basic lining costs more than the acid lining and does not last 
so long, the high phosphorus ores and scrap cost less than the 
ores required for the acid process. 

Silicon is desirable in small amount, to get rid of blow holes. 

4. Comparison of Materials.—When steel was first used to any 
considerable extent for structures, in the 1880’s, many engineers 
were doubtful of it and thought it an unreliable material, prefe- 
ring wrought iron, because the latter was softer and more ductile, 
because it was fibrous and therefore less likely to have a micro¬ 
scopic crack extend clear across the piece, and because it did not 
harden when suddenly cooled and was therefore less liable to 
injury in heat treatment or mechanical working. Since that time 
steels of all grades have come into use, soft and ductile if desired, 
and having strength considerably above that of wrought iron, 
thus effecting a saving of material. Wrought iron is now mainly 
used where welding is required, as it welds better than steel; and 
where special resistance to corrosion is desired, as many engineers 
believe that it resists corrosion better than steel. It is thus used 
for wire, for pipe, as a covering for roofs and sides of buildings, 
etc. Almost all structural work and machinery is made of steel. 

When steel was first used for structures, it was only in struc¬ 
tures of large size and in certain members, such as chords and 
eye bars, to attain economy by using a material of high ultimate 
strength and elastic limit, and so reducing weight. Steel was 
then much more expensive than wrought iron. Soon it was 
recognized that this high steel was not well suited to resist 
impact, and not a proper material for railroad bridges, and the 


STEEL 


93 


required strength was lowered while the ductility was increased. 
The extensive use of steel in structures did not commence till 
about 1890, when some railroads began to use it for spans of 
medium or short length. Schneider 1 gives the following table 
showing large bridges in which steel was used prior to 1899. 


Important Bridges of Steel, Prior to 1899 


Year 
of com¬ 
pletion 

River 

Description 

Ultimate 
strength, lbs. 
per sq. in. 

Minimum] 
elongation 
per cent 

1874 

Mississippi 

Arches, St. Louis. 

100,000 min. 

18 

1880 

Missouri 

Plattsmouth Bridge. 

80,000 min. 

12 

1882 

Missouri 

Bismarck: 





Comp. 

80,000-90,000 

12 



Tension. 

70,000-80,000 

18 

1882 

East River 

Brooklyn Bridge, excl. wire. 

70,000 min. 


1883 

Niagara 

Cantilever. 

80,000 min. 

15 

1884 

Susquehanna 

B. & O. R. R.: 





Comp. 

80,000 min. 

15 



Tension. 

70,000 min. 

18 

1885 

Arkansas 

Van Buren Bridge: 





Comp. 

80,000 min. 

15 



Tension. 

70,000 min. 

18 

1885 

Ohio 

Ky. and Ind. Cantilever: 





Comp. 

80,000 min. 

15 



Tension. 

70,000 min. 

18 

1886 

Harlem 

Washington Arch, New York. 

62,000-70,000 

18 

1887 

Missouri 

Sibley Bridge, A. T.& S. F.: 





Comp. 

75,000-85,000 

18 



Tension. 

60,000-70,000 

23 

1888 

Missouri 

Omaha: 





Comp. 

80,000 min. 

15 



Tension. 

70,000 min. 

18 

1888 

Mississippi 

Cairo Bridge. 

67,000-75,000 

20 

1888 

Ohio 

C. & 0. Ry. Cinn. & Covington: 





Comp. 

64,000-72,000 

19-17 



Tension. 

58,500-66,500 

20-18 

1890 

Firth of Forth 

Cantilever, Scotland: 





Comp. 

76,000-83,000 

17 



Tension. 

67,000-74,000 

20 

1890 

Colorado 

Red Rock Cantilever: 





Comp. 

64,000-72,000 

19-17 



Tension. 

58,500-66,500 

20-18 

1890 

Mississippi 

Merchants Bridge, St. Louis. 

67,000-75,000 

20 

1891 

Ohio 

Cantilever Highway, Cincinnati... 

62,000-72,000 

22 

1892 

Mississippi 

Memphis Cantilever: 





Main trusses. 

69,000-78,500 

18 



The rest. 

64,000-72,500 

22 

1893 

Mississippi 

Bellefontaine. 

62,000-70,000 

22 

1895 

Delaware 

P. R. R. Phila.: 





Main trusses. 

62,000-70,000 

22 



The rest. 

50,000-60,000 

26 

1897 

Niagara 

Double Deck Bridge. 

60,000-68,000 

20 

1898 

Niagara 

Highway Arch. 

60,000-68,000 

20 


* Erne. A.S.T.M ., 1902, p. 63. 














































94 


STRUCTURAL ENGINEERING 


By 1894, wrought iron shapes were practically unobtainable, 
and two grades of steel were in use; medium steel by those who 
advocated steel, and soft steel by those who perhaps preferred 
wrought iron, but being unable to obtain it, desired a material as 
nearly like it as possible. Present practice is quite uniform and 
has been standardized. The usual requirements will be given 
later. 

Open-hearth steel is very generally considered superior to 
Bessemer steel mainly because the open-hearth process is more 
easy to control, because less recarburizing is required, and because 
open-hearth steel contains less gases, which are injurious. Besse¬ 
mer steel is generally believed to be liable to be brittle and 
unsound, with less resistance to shock. Thus specifications for 
bridges, locomotives, cars, ships, axles, etc., require open-hearth 
steel; in steel buildings and rails, however, Bessemer steel is 
permitted (see table following). 


Materials Specified by A.S.T.M. 


Material 


Process specified 


Carbon steel rails (1914). 

O.H. steel girder and high tee rails (1921). 

Low-carbon steel splice bars (1914). 

Medium-carbon steel splice bars (1914). 

High-carbon steel splice bars (1914). 

Extra-high-carbon steel splice bars (1914). 

Quenched high-carbon steel splice bars (1921). 

Quenched carbon-steel track bolts (1921). 

Quenched carbon-steel track bolts nuts (1921). 

Quenched alloy-steel track nuts (1921). 

Quenched alloy-steel track bolts nuts (1921). 

Steel track spikes (1918). 

Steel screw spikes (1921). 

Structural steel for bridges (1921). 

Structural nickel steel (1921). 

Structural steel for buildings (1921).. 

Structural steel for buildings (rivets, pis. Ls over % inch to 

be punched). 

Structural steel for locomotives (1921). 

Structural steel for cars (1921). 

Structural steel for ships (1921). 

Rivet steel for ships (1921). 

Carbon steel bars for railway springs (1916). 

Carbon steel bars for railway springs (special silicon req.) (1918) 
Carbon steel bars for vehicle and automobile springs (1916). . 
Silica-manganese steel bars for automobile and railway 

springs (1916). 

Chrome vanadium-steel bars for automobile and railway 
springs (1916). 


Bes. or O.H. 

O.H. 

Bes. or O.H. 

O.H. 

O.H. 

O.H. 

O.H. 

O.H. 

Bes. or O.H. 

O.H. or el. furnace 
Bes. or O.H. 

Bes. or O.H. 

Bes. or O.H. 

O.H. 

O.H. 

Bes. or O.H. 

O.H. 

O.H. 

O.H. 

O.H. 

O.H. 

O.H., crucible, or el. furnace 
O.H., crucible, or el. furnace 
O.H., crucible, or el. furnace 


O.H., crucible, or el. furnace 
O.H., crucible, or el. furnace 


Billet steel concrete reinforcement bars (1914) 


Bes. or O.H. 

































STEEL 


95 


Materials Specified by A.S.T.M. —( Continued) 


Material 


Process specified 


Carbon-steel and alloy-steel blooms, billets and slabs for 

forgings (1921). 

•Carbon-steel and alloy-steel forgings (1921). 

Quenched and tempered carbon-steel axles, shafts, forgings 

for locomotives and cars (1921). 

Quenched and tempered alloy-steel axles, shafts, forgings for 

locomotives and cars (1921). 

Carbon-steel forgings for locomotives (1921). 

Carbon-steel cars and tender axles (1918)..,. 

Cold-rolled steel axles (1921).. 

Wrought solid carbon-steel wheels (steam railway) (1916).. . 
Wrought solid carbon-steel wheels (electric railway) (1916).. 

Steel tires (1916)... 

Steel castings (1921). 

Lap-welded and seamless steel boiler tubes for Iocs. (1921). . 
Lap-welded and seamless steel boiler tubes for stationary 

service (1918). 

Welded and seamless steel pipe (1921). 

Automobile carbon and alloy steels (1921). 

Boiler and fire box steel for Iocs. (1921). 

Boiler rivet steel (1921). 

Cold-drawn Bes. steel automatic screw stock (1914). 

Cold-drawn open-hearth steel automatic screw stock (1915). 
Commercial bar steel (1921). 


O.H. or el. furnace 
O.H. or el. furnace 

O.H. or el. furnace 

O.H. or el. furnace 
O.H. or cl. furnace 
O.H. 

O.H. or el. furnace 
O.H. 

O.H. 

O.H. 

O.H. el. furn., cruc. or side- 
blow converter 
O.H. 


O.H. 

Bes. or O.H. 

O.H., crucible, or el. furnace 
O.H. 

O.H. 

Bessemer 

O.H. 

Bes. or O.H. 


Acid and basic open-hearth steel are allowed in most specifi¬ 
cations. Formerly many engineers preferred acid open-hearth, 
because of danger of dissolved gases and irregularities of structure 
due to recarburizing basic steel after it leaves the furnace; and 
because the acid process does not remove phosphorus, and 
therefore requires a better cast iron to start with, which was 
considered safer than to use a cast iron with more phosphorus 
and depend on removing it. In modern specifications both 
processes are allowed, but the allowed percentages of sulphur 
and phosphorus are specified, the phosphorus being lower in 
basic steel. For the cables of the Delaware River bridge at 
Philadelphia (1923) acid open-hearth steel is specified with the 
following requirements: 

Carbon, not more than 0.85 per cent, 

Phosphorus, not more than 0.04 per cent, 

Sulphur, not more than 0.04 per cent, 

Tensile strength, per square inch of gross section, not less than 
215,000 pounds, 

Yield point per square inch of gross section, not less than 
144,000 pounds, 

























96 


STRUCTURAL ENGINEERING 


Elongation in 10 inches while under tension, not less than 4 
per cent. 

The following gives the specifications for steel for the Sydney 
Harbor bridge (1922) for plates and shafts up to and including 
1 inch in thickness: 


Specifications for Sydney Harbor River Bridge 



Carbon steel 

Nickel steel 

Chrome-nickel 

steel 

Minimum tensile strength, in 




pounds per square inch. 

62,000-70,000 

85,000-100,000 

85,000-100,000 

Minimum yield point, in pounds 




per square inch. 

35,000 

50,000 

50,000 

Percentage of elongation in 8 

1,500,000 

1,600,000 

1,600,000 

inches. 

Tensile strength 

Tensile strength 

Tensile strength 

Minimum reduction of area, 




percentage. 

44 

40 

30 


Trans. A.S.C.E. Vol. Ixxxvi , 1923, p. 1298. “The Study of Steels for Engineering 
Structures,” by George K. Burgess. 


Crucible steel is the best and most expensive kind, and is used 
for tools, and sometimes for springs. It is the best, because, 
like cementation steel, it is made in closed vessels, out of contact 
with the air. 

Steel refined in the electric furnace is also used for some 
purposes, especially in producing alloy steels and high-grade 
castings. 

5. Strength of Steel.—The strength of steel is very different 
according to the composition and the constitution, that is, 
according to the percentage of carbon and other elements and the 
heat treatment and mechanical working to which it has been 
subjected. Some alloy steels have very great strength. Increase 
of strength is generally accompanied by decrease of ductility. 
Carbon produces the greatest increase of strength with the least 
decrease of ductility. Alloy steels will be referred to in Chap. 
VIII. 

With increasing carbon, the tensile strength of normalized 
steel increases until the eutectoid composition is reached, about 
0.85 per cent C, because, being 100 per cent pearlite the grain is 
finest at that composition. With less carbon there is more free 
ductile ferrite, and with more carbon there is more free brittle 
cementite. This explains why ductility decreases as carbon 
increases. The books referred to in Chap. I give formulae 
















STEEL 


97 


for the increase of tensile strength due to each increase of 0.01 
per cent of carbon and other elements, but this supposes the same 
heat treatment in all cases, or normalized steel. Notwithstand¬ 
ing the great variation in strength, it is remarkable that the 
modulus of elasticity of all grades of steel remains nearly constant 
at about 30,000,000 pounds per square inch. 

Sauveur gives Fig. 31a to illustrate the relation between tenacity, 
elongation and pearlite, in annealed (pearlite) steels. 


50 . 125.000 

| .E 

| 40 jfioqooo 

CVJ !_ 

75,000 

s* rr> 

*>20 5QOOO 

.2 e 


§>10 

o 

^ 0 


25000 








~~1 





& 








xl 

f 






^ m 

, 

/ 










/ 


- 








Z 











°/o Carbon 0 
°lo FejC 0 
°/oPearlite 0 


0.4 

6 

48 


0.8 

12 

90 


93.0 90.2 


1.6 

24 

80.8 


too 


18 

27 

834 


W, 


2.0 

30 

80 


20 °- 

0 


Fig. 31a.—Diagram showing the relation between the tenacity and ductility 
of annealed (pearlite) steels and the carbon content. ( Sauveur , Metallography of 
Iron and Steel , 1912, p. v, 18.) 


The tensile strength of carbon steel, depending on the different 
factors named, may be from 50,000 to over 150,000 pounds per 
square inch; and the stress-strain curve may vary from nearly 
that of wrought iron, with a well-defined yield point at which a 
large stretch occurs, to a line with practically no yielding at the 
so-called yield point (which in this case is very indefinite), curv¬ 
ing gradually from the proportional limit up to the breaking 
strength. 

Thus, Fig. 32 shows stress-strain diagrams for round rods 
before drawing into wire, with an area of 0.036 square inch, and 
the finished square wire with an area of 0.01 square inch. The 
rods themselves had probably been subjected to some mechan¬ 
ical cold working, of course, to reduce them to a diameter 
of 0.216 inch, so that the strength of the rods was above that of 
the same material normalized to remove the effect of mechanical 
working. The wire drawing was cold working, and the curves 
show the effect of this. 

Steel does not stretch so uniformly over its entire length as 
wrought iron, under a tensile stress, but necks down more. 






















98 


STRUCTURAL ENGINEERING 


The table on pages 100 and 101 gives the physical properties 
of carbon steel used for various purposes, as specified in the 1921 
Book of Standards of the A.S.T.M.: 



The specifications for the Blackwell’s Island bridge contained 
the following requirements (1904): 


Material 

Physical properties 

Structural nickel steel 

Structural carbon steel 

Eye-bars 

unannealed 

Ultimate tension 
Elastic limit 

Elongation in 8 inches 

100,000 

55,000 

1,600,000 

66,000 
ultimate 

1,500,000 

Ultimate tension 

Ultimate tension 

Pins 

unannealed 

Ultimate tension i 
Elastic limit 

Elongation 

90,000 

55,000 

20 per cent in 2 inches. 

66,000 

H ultimate 
1,500,000 . , 

Ultimate tension in lnc 1CS 












































STEEL 


99 


The ultimate compressive strength of steel depends upon its 
composition and constitution, and the conditions of use. As 
shown in Chap. XXIII of the “Strength of Materials/’it is impos¬ 
sible to conceive of the failure of an engineering material, or of any 
material, in pure compression unaccompanied by other kinds of 
stress, unless failure is taken to occur when the compressive 
deformation reaches a certain limit. Especially is this true 
of a material like steel. Short blocks in compression will fail 
by shearing or bulging; it is not pure compression, and defor¬ 
mation limits. A long column will fail by buckling of some 
part or of the column as a whole; in this case the ultimate 
strength depends upon the character of the material. If it 
is ductile, showing a large yielding at the yield point, in com¬ 
pression, the ultimate strength will be at the yield point or a 
little above. This, however, is a combination of compression 
and bending, accompanied by eccentricity of load. Even if the 
column as a whole remains straight until the yield point is 
reached, when that point is reached a flow of metal will take place, 
and some part, such as a thin outstanding leg, will be buckled, 
producing local eccentricity, and failure will occur, perhaps in 
the end by a tension break. As the material becomes harder, 
and shows less yielding at the yield point, the ultimate will rise, 
until for very hard material it may be considerably above the 
elastic limit; the yield point is here uncertain. 

The shearing strength of steel is found by experiment to be not 
far from 0.8 the tensile strength. 

The torsional strength is the shearing strength, but the formula 
for torsion will not give the ultimate strength equal to the true 
ultimate shearing strength because in torsion the shear is not 
distributed uniformly. This topic has been discussed in Art. 
33 of Chap. X of the previous volume. The real stress at the 
outer fiber is less than that given by the torsion formula, by an 
amount depending upon the shape of the actual stress-strain 
curve under these conditions, and this is unknown. 

The flexural strength is a combination of tension, compression, 
and shear, and involves the shape of the section. This subject 
has been referred to in Chap. X of the previous volume, and will 
be further treated in Vol. 3, in discussing the design of beams. 

6. Cold Bending. —The ductility of steel is indicated by its 
ability to bend cold, without cracking, either flat upon itself or 
around a pin having a diameter depending upon the thickness of 



Specifications of the A.S.T.M., 1921 


100 


STRUCTURAL ENGINEERING 


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102 


STRUCTURAL ENGINEERING 


the piece. The specifications of the A.S.T.M. for steel for various 
purposes are given in the following table: 


Standard Specifications for Cold-bending Tests of Various Steels 


Character and use of steel 


Cold bending 


Angle, 

degree 


Diameter 1 


Bridge, building, car ( Not over 24 inch thick. 

and locomotives 34 inch to 134 inches thick. 

structural steel { Over 134 inches thick. 

Rivet steel for bridges, buildings, boilers, ships, etc. 

{ Not over 24 inch thick. 

24 inch to 134 inches thick. 

Over 134 inches thick. 

Soft steel castings (specimens are 1 inch by 34 inch section). 

Medium steel castings (specimens are 1 inch by 34 inch section). 
Boiler and firebox J Not over 1 inch thick. 


steel. 

1 Over 1 inch thick 



Structural 

Under % inch thick. 



grade 

% inch thick or over. 

Billet 

steel 

concrete 

Plain bars 

< 

Intermediate 

grade 

Hard grade 

Structural 

Under ^4 inch thick. 

1 % inch thick or over. 

Under % inch thick. 

24 inch thick or over. 

Under 24 inch thick. 



reinforcing 


grade 

1 24 inch thick or over. 

bars 

Deformed 

bars 

Intermediate 

Under 24 inch thick. 


grade 

24 inch thick or over. 



Hard grade 

Under 24 inch thick. 

1 24 inch thick or over. 

Cold-twisted bars 


Under 24 inch thick. 




24 inch thick or over. 



Plain bars J 

Under 24 inch thick. 

Concrete 

reinforcing 


24 inch thick or over. 

bars from rerolled rails 

Deformed 

Under 24 inch thick. 



bars 

24 inch thick or over. 


180 

180 

180 

180 

180 

180 

180 

120 

90 

180 

180 

180 

180 

180 

90 

ISO 

90 

180 

180 

180 

90 

180 

90 

180 

180 

180 

90 

180 

90 


0 

t 

2 t 
0 

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in. 

in. 

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t 

t 

t 

t 

t 

t 

t 

t 

t 

t 

t 

t 

t 

t 


1 Diameter of pin about which specimen of thickness t is bent. 


If a piece of thickness t be bent around a pin of diameter d, and the inside 

• TV (J/ 

of the piece around the bend does not change its length the outside, 
which had originally this same length, is stretched to a length ^ 

£ 

2 1 

or is stretched a distance irt ; or the stretch per unit length is -j. The 

severity of the test depends, therefore, upon the ratio and for uniformity 

this ratio should be constant. Generally, however, the material on the 
inside of the bend will be compressed. 












































STEEL 


103 


7. Steel at High Temperatures. 1 —For some purposes, as for 
boilers, and for structural steel in a conflagration, the properties 
at high temperatures are of importance. 

Figures 33, 34 from “Tests of Metals,” 1888, shows the results 
for tensile strength. The strength slightly decreases up to about 
300° F. for high carbon steels; at about 600° F. the strength is a 
maximum and some 10 to 15 per cent above that at normal 
temperature; from this point it decreases rapidly until at about 
800° C. it is small, as might be expected. 



Fig. 33.—Variations in tensile strength of ferrous metals with temperature. 
(Tests of Metals, 1888.) 


Cast iron is reduced in strength less than wrought iron or steel 
being not materially affected up to about 550° C., above which it 
falls, though not as rapidly as wrought iron or steel. 

The modulus of elasticity decreases at high temperatures, as 
would be expected. 

At a blue heat (500 to 600° F.) the ductility and toughness of 
wrought iron and steel is much reduced . 2 Bars which at normal 
temperatures could be bent back and forth 12 to 26 times 
through an angle of 45 ° first on one side and then on the other of 
the straight position, withstood at a blue heat only lj^ to 3 bends. 
The resistance to impact is also least at this temperature (see 
also Art. 12 , Chap. III). Annealing removes these effects. 

1 Johnson’s “Materials,” by Withey and Aston, Chap. XXVII: Tech¬ 
nologic Papers of the U. S. Bureau of Standards No. 219; Watertown Arsenal 
Tests of Metals, pp. 243-323, 1888. 

2 Strohmeyer in Proc. Inst. C. E., vol. 84, p. 114. 































104 


STRUCTURAL ENGINEERING 


At very low temperatures the strength and the elastic limit are 
increased, while the ductility is decreased in even greater propor¬ 
tion. The resistance to impact appears to decrease as the tem¬ 
perature falls below normal; the number of blows required to 
break a wrought-iron axle at 100° F. being found to be 50 per 
cent greater than the number required at 0° F. Rails are sup¬ 
posed to be more brittle in very cold weather than in warm 
weather, there being more rail fractures in winter than in sum¬ 
mer; though this might be due to the fact that the frozen roadbed 
is more rigid than in summer. 



Temperature 


Fig. 34.—Grand mean curves from temperature tests on steel rods. Diameter 
of rods \}/± inch; of specimen 0.8 inch. Ten degrees of hardness, from 0.09 to 0.97 
per cent C. (Test of Metals, 1888, p. 245.) 

8. Steel Castings.—A steel casting does not receive the benefit 
of mechanical work, and is therefore likely to be more unsound, 
containing more dissolved gases, and more blow-holes, than forged 
or rolled material. The usual defects of castings are: (1) Blow¬ 
holes or gas bubbles; (2) a pipe, or cavity formed near the center 
or top of an ingot by the cooling of the outer surfaces and subse¬ 
quent shrinkage as the remaining liquid cools; (3) ingotism, or 
the formation of large crystals, that is, coarse grain; (4) Segrega¬ 
tion, or concentration of the impurities in the most easily fusible 
metal part which solidifies last; (5) Shrinkage stresses, which 
sometimes produce cracks. 

























STEEL 


105 


Blowholes are liable to form in steel castings, especially in low- 
carbon steel, as air bubbles form in ice. They are particularly 
liable to occur in Bessemer steel, especially when the blast enters 
at the bottom and passes through the entire body of metal. 
Sometimes the blast enters at the side (side-blow converters) 
through the trunnions, and in this case blow-holes are less likely 
to occur. Manganese and silicon, added in the recarburizer, 
combine with the oxygen and go into the slag. 

Blow-holes in ingots which are afterward worked, as by 
rolling, are flattened out, and the surfaces, if not oxidized, are 
welded together and the gas forced out. In castings there is no 
such action, and the blow-hole remains. 

A pipe generally occurs near or at the top of an ingot, and the 
top is cut off and discarded in order to remove it. A steel cast¬ 
ing generally has a riser or feeder which contains liquid metal that 
runs down and feeds any cavity that may tend to form. This 
riser is cut off after the casting has cooled. Cast iron does not 
have a pipe because it expands on solidification, as shown in 
Chap. IV. 

Ingotism or large crystallization, is sure to occur in castings to 
some extent. It is especially likely to occur in harmful degree if 
the steel is cast at too high a temperature or is allowed to cool too 
slowly. It may be regulated to some extent by skilful treatment. 
It is corrected by mechanical work on rolled or forged material, 
but it remains in castings, though annealing under proper condi¬ 
tions may largely remove it. 

Segregation to a certain extent cannot be prevented, though it 
is believed that the addition of aluminum will reduce it. It 
depends somewhat on the shape of the casting. A high casting 
temperature heats the mold, and results in slow cooling of the 
casting, with consequent coarse structure and perhaps segrega¬ 
tion; hence the casting temperature should be low. This will 
also decrease the shrinkage stresses. 

Shrinkage stresses will always exist, but they may be reduced 
by proper design of the castings. Annealing will largely remove 
them. 

It is obvious that steel castings are less reliable than forged or 
rolled material. The specifications of the A.S.T.M. require, if 
Bessemer steel is used, that it shall be from a side-blow converter. 
Both acid and basic steel are allowed, although acid steel is gener¬ 
ally thought to be better for castings than basic steel. This is 


106 


STRUCTURAL ENGINEERING 


because acid steel starts with a pig lower in sulphur and 
phosphorus than basic steel, hence there is less segregation; and, 
because of the easier recarburizing of acid steel and the more 
uniform dissemination of the recarburizer, there is apt to be less 
danger of blowholes. Steel castings in which strength is 
important should always be annealed. 

9. The table on preceding page, from paper by Dr. Geo. K. 
Burgess, Director of the U. S. Bureau of Standards, in the Trans. 
A.S.C.E., 1923, gives some structural steel specifications. 


Structural Steel Specifications 


STEEL 


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CHAPTER VIII 


ALLOY STEELS 1 

1. We have seen that steel is an alloy of iron with carbon and 
small quantities of other elements. The essential ingredient is 
carbon, and such steel is called “carbon steel”; the other 
ingredients being either those which cannot be eliminated, or 
which are purposely added in small quantities to produce some 
benefit without greatly changing the properties; as silicon to get 
rid of blowholes, and manganese to promote soundness by neu¬ 
tralizing the bad effect of sulphur and to eliminate oxygen. An 
alloy steel is one which owes its distinctive properties chiefly to 
the presence of an element or elements, other than carbon, or 
jointly to such element and carbon. If one other element is 
added, the alloy is a ternary alloy; if two are added, it is a quater¬ 
nary alloy. 

2. The principal ternary alloys are with nickel, manganese, 
chromium, tungsten, silicon, vanadium, molybdenum or tita¬ 
nium. The quaternary alloys are chrome-nickel, chrome- 
vanadium, and many others. There are alloys with more than 
four main ingredients, called complex alloys. 

There are obviously innumerable alloys, depending on the 
proportions of the ingredients. The subject is very complicated, 
and rapidly changing as knowledge is gained. New alloys are 
being introduced, the character of some being kept secret. 

3. The alloying elements in some cases form solid solutions 
with iron, while in others they form chemical combinations or 
liquid solutions which separate on freezing. A small proportion 
of an ingredient may produce very great effects, and in different 
proportions it may confer entirely different, or even opposite, 
qualities. 

1 The student should read the chapters on this subject in Stoughton, 
Upton, Mills and Johnson; also Hibbard’s short book on the “Manu¬ 
facture and Uses of Alloy Steels.” The paper by Geo. K. Burgess, 
Director of the Bureau of Standards, in Trans. A.S.C.E ., pp. 1292-1315, 
1923, should also be studied. 


108 


ALLOY STEELS 


109 


In most cases the alloying element depresses the critical tem¬ 
perature, at which austenite is transformed. Some have the 
effect of allowing austenite or martensite to be produced by slow 
cooling, while in carbon steel we have seen that these forms can 
only be obtained in the cold metal (if there is sufficient carbon) 
by very rapid cooling. Martensite produced by slow cooling is 
more stable than when produced by rapid cooling, for in the 
latter case the normal transformation is incomplete, having been 
arrested or prevented by the quenching. Nickel and manganese 
act in this way, and chromium to a less degree. This renders heat 
treatment easier in these alloy steels. If an alloy steel, cooled 
slowly, is martensitic in structure, heating and quenching it 
may make it austenitic, that is, may soften it, and the ductility 
and strength may be much increased. 

If a hard steel is desired, it is an advantage if it can be 
produced by slow cooling, for heat treatment is thus rendered 
unnecessary. Alloy steels which become hard when cooled 
slowly are called “self-hardening steels.” 

4. The properties of alloy steels are very dependent upon heat 
treatment. By varying this treatment, a steel may be given a 
wide range of properties. The effect of heat treatment is illus¬ 
trated by the following tests on pieces of steel which were all 
from the same heat but had been subjected to different heat 
treatment. 1 


Tensile strength 

Elastic limit 

Ratio of 
elastic limit 
to tensile 
strength 

Elongation 
in 2 inches, 
per cent 

Contraction 
of area, 
per cent 

84,850 

50,000 

0.60 

28.0 

67.5 

120,975 

90,000 

0.745 

14.5 

51.0 

166,950 

157,500 

0.94 

12.5 

44.0 

205,600 

200,000 

0.97 

13.0 

48.7 

240,975 

225,000 

0.93 

9.0 

20.5 


5. Manganese Steel.—Manganese, as we have seen, normally 
occurs in small quantities in most steels, promotes deoxidation 
and soundness, and makes sulphur practically harmless. If, 
however, when combined with high carbon, it is increased to from 

1 Hibbard, p. 7. 











110 


STRUCTURAL ENGINEERING 


5 to 10 per cent, it makes an alloy which has very little ductility, 
and is too hard and brittle (martensitic) for use; but if from 12 
to 14 per cent, with carbon about 1 per cent, it produces an alloy 
which, with proper heat treatment (heating and quenching) 
combines ductility with great strength and toughness. This is 
much used to resist wear, and generally must be cast, as it is too 
hard to be rolled or machined. These facts show that if a certain 
percentage of an element produces certain effects, it does not 
follow that a larger percentage will produce the same effects in 
greater degree; it may produce opposite effects. 

Manganese steel has not, in the knowledge of the writer, been 
used in structures. Its strength is much greater than that of 
structural steel (soft or medium carbon steel) but its elastic 

limit is lower in proportion 
to its ultimate strength. It 
could not be used for struc¬ 
tural members; but it is 
used for parts where great 
hardness is desired, as, for 
the jaws of rock crushers, 
and in some cases for rail¬ 
road rails subject to spec¬ 
ially heavy wear and for 
frogs and switches, and 
burglar-proof safes. 

In the Boston subway 
cast manganese rails were 
placed in 1902 on a curve of 82 feet radius, to replace low-carbon 
steel rails; they lasted 14 years where the carbon steel rails had 
shown an average life of 44 days, with about the same wear. 
The manganese rails had 1.25 per cent C and 12 per cent Mn. 

Manganese steel cannot be softened by heating followed by 
slow cooling. The constitution is shown by Fig. 35, which shows 
that without quenching it may be pearlitic, martensitic, or 
austenitic. It has the curious quality of stretching very uni¬ 
formly throughout the length of a test piece. “In the pulling 
test the percentage of contraction of area is less than the elonga¬ 
tion, a result directly opposite to that with simple as well as most 
alloy steels, in which the percentage of contraction is usually 
twice or more that of the elongation. The pulled test piece has 
a rather uniform stretch throughout its length, whereas carbon 



Fig. 35. —Constitution of manganese 
steels. (Guillett Jour. Ir. and St. Inst, vol 
70, p. 7.) 










ALLOY STEELS 


111 


steels, as is well known, have a largely increased amount of stretch 
near the point of fracture. When a piece of manganese steel is 
pulled, the increase of strength due to cold working (stretching) 
is greater than the decrease in cross-section due to contraction, 
so that a stretched part becomes stronger than the unstretched 
parts, and elongation then occurs in another place. As the pull¬ 
ing is continued, all parts of the pulled section stretch one after 
the other, with the result that when the piece is finally ruptured 
the stretch has been comparatively uniform. There is indeed 
an increased local extension and contraction close to the point of 
rupture, as with other ductile steels, but it is less marked in the 
manganese steels.” 1 

Manganese steel is generally toughened by a heat treatment by 
heating it to about 1,050° C. and quenching in cold water. 

Hibbard gives the following results of a test of cast, heat-treated 
manganese steel, forged down to the size given: 

Diameter, 0.823 inch; length, 2 inches; 

Per cent C, 1.10; Mn, 12.4; Si, 0.15; P, 0.06; 

Tensile strength, 152,840; elastic limit, 56,400; 

Per cent elongation, 51.0; contraction of area, 39.5. 

The toughness, or resilience, or resistance to shock, may be 
roughly measured by the product of the ultimate strength and the 
elongation. This has been called the “merit number.” In 
the case of the above steel it is 7,794,840. Hibbard states that 
the merit number of manganese steel (Mn 11 to 14 per cent) is 
“perhaps the greatest of all known steels.” He gives the follow¬ 
ing table: 


Merit Numbers of Various Metals 


Metal 

Tensile 
strength, 
pounds per 
square inch 

Elongation, 
per cent 

Merit 

number 

Manganese steel. 

140,000 

60,000 

50.0 

7,000,000 

Soft steel. 

30.0 

1,800,000 

Tool steel. 

130,000 

5.0 

650,000 

Cast iron . 

20,000 

0.5 

10,000 

Nickel steel natural. 

95,000 

207,000 

21.0 

1,995,000 

Nickel steel, heat-treated- 

14.0 

2,898,000 


1 Hibbard. 

















112 


STRUCTURAL ENGINEERING 


Of course the accurate ultimate resilience depends upon the 
shape of the stress-strain diagram, as explained in Chap. IV of the 
previous volume. 

The heat treatment described for manganese steel increased its 
tensile strength from about 88,000 to 145,000 pounds, its elonga¬ 
tion from 3.5 to 50 per cent, and its merit number from 308,000 
to 7,262,000, according to certain tests. The importance of heat 
treatment is obvious. 

The relatively low elastic limit of manganese steel is noticeable. 
Remarkable, also, is the fact that sudden cooling, which produces 
brittleness in carbon steel, produces ductility in manganese steel. 

Manganese confers a fine grained structure, even after slow 
cooling; and it shifts the eutectoid point toward the lower carbon 
content. One per cent of manganese is said to lower the eutec¬ 
toid ratio to 0.78 per cent carbon. 1 

6. Nickel Steel.—Nickel steel is the most valuable alloy for 
the structural engineer, sometimes as a quaternary alloy with 
chromium (chrome nickel steel). Chromium is cheaper than 
nickel, and the chrome-nickel steels are increasing in favor. 

Nickel does not occur in ordinary carbon steel, nor does it affect 
such steel in any beneficial way, as manganese and silicon do. It 
is added solely to confer desired qualities upon the alloy. These 
qualities depend not only on the amount of nickel, but also upon 
the carbon and the heat treatment. Simple nickel steel for 
structural purposes contains from 2 to 4 per cent nickel; generally 
about 3.25 is meant when nickel steel is mentioned without 
further qualification. Such steel, with C. 0.25 and Mn about 
0.86, will have, without heat treatment after rolling, a tensile 
strength of about 100,000 pounds and an elastic limit of nearly 
70,000, with little reduction of ductility as compared with 
structural carbon steel. Perhaps its most valuable structural 
quality is that the elastic limit and the yield point are raised in 
greater proportion than the ultimate strength. In ordinary 
carbon steel the elastic limit should be at least half the ultimate 
strength, but in 3.5 per cent nickel steel it should be at least six- 
tenths. Thus, if the ultimate is increased 80 per cent, the elastic 
limit should be over twice that of structural carbon steel, 2 and 

1 Scientific paper 464. U. S. Bureau of Standards. See also Scientific 
paper 453. 

„ 1.8 X 0.6 



ALLOY STEELS 


113 


the allowable unit stress may be increased in about the same 
proportion. Nickel steel is thus peculiarly applicable in the case 
of long-span bridges, in which it is important to reduce the dead 
weight. It has been used in the Blackwell’s Island, Hell Gate, 
and Manhattan bridges in New York, the Quebec bridge, the 
St. Louis Municipal bridge, and the Kansas City viaduct. The 
value of E for such steel is about the same as for carbon steel, 
about 29,500,000 pounds per square inch. With higher percent- 



Fig. 36.—Constitution of nickel steels. (Guillett Jour. Ir. and St. Inst., vol. 70 

V • 4.) 

ages of nickel, while the strength is greater, the modulus of 
elasticity may be lower, making the material less suitable for struc¬ 
tures. Nickel steel is harder than carbon steel, corrodes less, and 
has a lower melting point, so that it is very suitable for castings. 
On the other hand, nickel hinders the welding of steel, so that if 
there are blowholes they are less apt to be welded up in rolling, 
but are more likely to be drawn out into injurious seams; but if 
oxidized they will probably be so drawn out in any steel. 

Guillet’s diagram of the constitution of nickel steel, not heat 
treated, is shown in Fig. 36. Pearlite is strong and ductile; 
martensite stronger and more brittle; austenite stronger than 












114 


STRUCTURAL ENGINEERING 


pearlite but less strong than martensite, and at the same time 
tough and ductile. Hence it would be expected that low nickel 
steels (pearlitic) and high nickel steels (austenitic) would be the 
most useful except where great hardness is desired; and since 
nickel is expensive, low nickel steels are those most used. The 
modulus of elasticity of high nickel steel is lower than that of low 
nickel steel (only about 23,000,000 pounds per square inch), 
hence the former yield more under stress, and are less suitable 
for structures even if the cost were not high. 

Nickel steel with 22 per cent nickel has been used for the valve 
stems of the salt-water fire protection service of New York, where 
it was essential that corrosion should be prevented. Generally 
speaking, the high nickel steels are only used where resistance to 
corrosion is very important (as in boiler tubes); or for their high 
electrical resistance. 

Owing to the lowering of the transformation temperature, 
nickel steel may therefore be pearlitic, martensitic, or austenitic, 
without quenching, depending upon the percentages of nickel 
and carbon. 

Alloys with nickel 10 to 25 per cent, are martensitic, and by 
quenching become austenitic, that is, are softened. With 
nickel about 20 per cent, the transformation temperature from the 
austenitic condition, on cooling, is lowered to below 100° C.; and, 
on reheating, the austenitic condition will not be resumed until 
a temperature of about 600° C. is reached, whereas in ordinary 
carbon steel the heating transformation point is only 25 to 50° 
above the cooling point. This means that the alloy may exist 
between 100 and 600° in either condition—martensitic or austen¬ 
itic. The two ends of the same bar may thus be made to possess, 
within those temperatures, quite different properties, one end 
being magnetic and the other not. With 20 per cent nickel, 1 
per cent carbon, and 1.4 per cent manganese, the transformation 
temperature on cooling is reduced to about 188° C. below zero; 
and on heating to above usual atmospheric temperature. With 
over 25 per cent nickel, the heating and cooling transformation 
temperatures are about the same. This phenomenon is expressed 
by the statement that nickel steels with less than 25 per cent 
nickel are irreversible, while those with more are reversible. 

“Invar” steel, containing 36 per cent nickel, has the peculiarity 
that it has a very low coefficient of expansion. It is used for steel 
tapes, clock pendulums, etc. 


ALLOY STEELS 


115 


With 42 per cent nickel, the metal, known as “platinite,” has 
the same coefficient of expansion as glass, and can therefore be 
used as wire in “wire glass” and in electrical connections through 
glass without cracking the glass under temperature changes. 

Nickel steel resists shock and fatigue better than carbon steel. 
It will no doubt be used more and more in structures. 

7. Chrome Steel. —Chrome steel, which is practically always 
heat-treated by quenching and drawing, is stronger and much 
harder than carbon steel with the same carbon content, and is also 
ductile and tough. The percentage of chromium does not often 
exceed 2 per cent and is generally 1 per cent or less. The elastic 
limit is nearly 90 per cent of the ultimate and sometimes more. 
Chrome steel is used in mining machinery, and for vaults, rolls, 
files, balls and rollers for bearings, armor plate, projectiles, and 
certain automobile parts which require hardness. It is largely 
giving way to chrome-nickel steel. 

8. Silicon Steel. —Silicon, as we have seen, is frequently added 
to steels to promote soundness by absorption of oxygen. The 
percentage in ordinary steel is usually below 0.3. If the percent¬ 
age is increased to from 0.3 to 0.5, a considerable increase of 
strength is obtained, with little loss of ductility. Silicon steel with 
not over 0.45 has been used in the new Delaware River bridge 
towers. This is a suspension bridge, the cables being of 0.85 
carbon steel with an ultimate strength of 215,000 and a yield 
point of 144,000 pounds per square inch. The specifications for 
the towers are given at the end of this chapter. 

For electrical purposes, a silicon steel developed by Hadfield 
has been used, which has very high magnetic permeability and 
electrical resistance, and low hysteresis. It contains 4 to 4.5 per 
cent of silicon, and the smallest possible amount of carbon, man¬ 
ganese, and other impurities, and is subjected to heat treatment by 
heating first to 1,070° C., cooling quickly to atmospheric tempera¬ 
ture, then heating to 750° C. and cooling slowly, afterwards 
sometimes again heating to 800° C. and cooling slowly. 1 

Silicon steel is also used for automobile springs, and will 
probably be more extensively used in structures. 

9. Copper Steel. —In Chap. XIII, reference is made to the use 
of steel with about 0.25 per cent copper, to resist atmospheric 
corrosion. 


1 Stoughton, p. 444. 


116 


STRUCTURAL ENGINEERING 


10. Vanadium in very small percentages (less than 0.2) and 
with suitable heat treatment, increases greatly the strength of 
carbon steel, especially against shock and alternating stresses. 
It is being more and more used, especially with chromium. 
Vanadium is a deoxidizer, and therefore promotes soundness. 
Chrome-vanadium steel is used extensively for automobile parts. 
It is more free from seams than alloys with nickel. 

11. Chrome-nickel steels are important structural materials. 
By suitable proportioning and heat treatment (they are 
practically always heat-treated) they “can be made to have as 
high physical properties as any steels known, with any 
elastic limit between 40,000 and 250,000 pounds per square inch, 
accompanied by ductility that is high as compared with its 
strength.” As chromium is cheaper than nickel, chrome-nickel 
steels can be made more cheaply than simple nickel steel of the 
same strength and ductility. 1 These steels are used in automo¬ 
biles, and for armor plate and projectiles. 

Mayari steel is a chrome-nickel steel from ore mined at Mayari 
in Cuba. It was first used in bridge construction in the Memphis 
bridge, by Ralph Modjeski, and has since been used in a number 
of bridges. It contains from 1 to 1.5 per cent nickel and 0.20 to 
0.75 per cent chromium. 

Chrome-vanadium steels are much used for automobile parts, 
and for some other purposes. They are much like chrome nickel 
steels, but lack the imperfection which nickel steels sometimes 
possess, namely the tendency to have seams if the ingots have 
blowholes. This is because vanadium is a deoxidizer, while 
nickel is not. 

12. High-speed Tool Steels, or “Rapid” Steels. —With 16 to 
20 per cent tungsten, 2 to 6 per cent chromium, carbon 0.5 to 0.7 
and a small amount of vanadium, a steel is produced which, with 
suitable heat treatment, will retain great hardness when red-hot. 
This alloy, and the method of heat treatment, were discovered 
by F. W. Taylor 2 and Maunsel White, of the Bethlehem steel 
works; and, used as a cutting tool it has revolutionized the art of 
cutting metals, making it possible to take heavier and more rapid 
cuts than formerly. Tungsten is considered to be the element 
that gives the steel hardness and toughness at a red heat, and its 
use makes the best percentage of carbon only about half that 

1 Hibbard. 

2 Taylor, F. W.: The Art of Cutting Metals; Trans. A.S.M.E ., 1906. 


ALLOY STEELS 


1 17 

required in cutting tools of carbon steel. The heat treatment 
consists in heating nearly to the fusion point, or line AD in Fig. 
11 and quenching in oil, with sometimes a second heating. No 
tempering change occurs on reheating to below 550° C., 
indicating a stable condition. The action of these steels is said 
to be due to the formation of a double carbide of chromium 
and tungsten. 

13. A steel is called “ self-hardening ” or “air-hardening” 
when it is hard and austenitic or martensitic when cooled slowly, 
that is, when the transformation point has been lowered to below 
atmospheric temperature. Self-hardening steels which remain 
austenitic are non-magnetic because the ferrite is in the 7 -form. 
Those which become martensitic, which is more usual, are 
magnetic. 

The action of alloying elements, even in small amounts, is 
often very peculiar. An alloy steel with 9 per cent tungsten, 
2.5 per cent manganese, and 1.85 per cent carbon is incapable of 
being made soft by any known process. The same result is 
produced if the manganese is replaced by 1 or 2 per cent 
chromium. This is peculiar, because tungsten alone does not 
reduce the critical temperature, while chromium alone raises it 
slightly, though the combination reduces it below atmospheric 
temperature. 

14. There follow extracts from the specifications for the new 
Quebec bridge (1913), and for the main towers of the Delaware 
River bridge (1922). 

The structural engineer will have little occasion to deal with 
alloy steels unless he has to build a very large bridge. In that 
case he should investigate the subject and obtain the counsel 
of a competent metallurgist. The subject is a specialized 
and intricate one. Up to the present time nickel steel, chrome- 
nickel steel, and silicon steel are the only alloys much used 
in structures. 

The mechanical engineer, especially if concerned with 
automobiles, will find it necessary to inform himself on the 
subject. 


118 


STRUCTURAL ENGINEERING 


SPECIFICATIONS FOR THE NEW QUEBEC BRIDGE (1913) 
Rolled Carbon Steel 

162. Furnace Used .—All structural steel shall be made in an open- 
hearth furnace. 

166. Chemical Requirements .—The ladle tests of steel as usually taken 
shall not contain more than the following proportions of the elements 
named: 



Acid, 

Per 

Basic, 

Per 


Cent 

Cent 

Phosphorus. 

. 0.06 

0.04 

Sulphur. 

. 0.04 

0.04 

Manganese. 

. 0.70 

0.70 

Except rivet steel. 


0.60 

No chromium to be used. 

Silicon. 

. 0.10 

0.10 


It is desired that the carbon contents be as small as possible to meet 
the specifications. 

167. Rivet Steel .—The ladle tests of the carbon rivet steel shall not 
contain more than 0.03 of 1 per cent of phosphorus, and not more than 
0.03 of 1 per cent of sulphur. 

170. Physical Requirements .—Specimens cut from the finished material 
shall show the following physical properties: 


Material 

Ultimate 
strength, 
pounds per 
square inch 

Minimum 
yield point, 
pounds per 
square inch 

Minimum 
elongation, per 
cent in 8 inches 

Minimum 
reduction, 
per cent 
of area 

Shapes and plates up to and 



1,500.000 

ultimate 


including 1 inch thick. . . . 

62,000 to 70,000 

35,000 

44 

Shapes and plates over 1 





inch thick. 

62,000 to 70,000 

33,000 

22 and 20 per cent 
for sheared plates 

40 

Eye-bar flats (unannealed). . 

66,000 to 74,000 

35,000 

22 per cent 
1,500,000 

40 

Rivets. 

48,000 to 56,000 

28,000 

ultimate 

50 

Pins and rollers (annealed). . 

65,000 to 75,000 

35,000 

22 per cent in 2 
inches. 

35 


Yield to be determined by drop of the beam. 

Speed of machine for testing samples to be such that material under 
tension wili not elongate more than 1 inch in two minutes. 





















ALLOY STEELS 


119 


171. Bending Tests. —Specimens cut from plates, bars and shapes 2 
inches wide shall bend cold 180° around a rod of a diameter equal to 
the thickness of the specimen; when at or above a red heat, 180° flat. 

Specimens cut from rivet rods shall bend 180° flat when cold, or when 
at or above red heat. A test piece 2 inches long when heated to a bright 
cherry red shall flatten longitudinally under the hammer to a thickness 
of one-quarter (34) inches without cracking on the edges. 

Full-sized sections of eye-bar material as rolled without annealing 
shall bend cold about a rod of diameter equal to twice the thickness of 
the bar. Angles of all thicknesses shall open cold to an included angle 
of 150° and close to an angle of 30° without a sign of fracture. 

All specimens in bending tests must show no signs of cracking on the 
outside of the bend. 

172. Fractures in Tension. —The fracture of all tension tests shall show 
a fine silky texture, of a uniform bluish grey or dove colour, free from 
black or brilliant specks, and show no sign of crystallization. 

Rolled Nickel Steel 

173. Furnace. —All nickel steel shall be made in an open-hearth fur¬ 
nace. It shall be made in the same manner as specified for rolled carbon 
steel, with the addition of nickel. 

174. Chemical Requirements. —The ladle test shall contain not less 
than 3.25 per cent of pure nickel, and not more than the following 
proportions of the elements named: 



Acid, 

Per 

Basic, 

Per 


Cent 

Cent 

Phosphorus. 

. 0.06 

0.04 

Sulphur. 

. 0.04 

0.04 

Manganese. 

No chromium to be used. 

. 0.70 

0.70 

Silicon. 

. 0.10 

0.10 

Carbon. 

. 0.45 

0.45 


175. Heating and Rolling. —Care shall be taken in the heating and 
rolling of nickel steel to prevent the formation of heavy scale. The 
material must not be pitted by rolling the scale into it. All material 
with pitted or heavily scaled surfaces, or with ragged edges, will be 
rejected. 

176. Physical Requirements. —Nickel steel for plates, shapes, and 
unannealed eye-bar flats must meet the following physical requirements 
in the finished material: 







120 


STRUCTURAL ENGINEERING 


Material 

Ultimate 
strength, 
pounds per 
square inch 

Minimum 
yield 
point, 
pounds per 
square inch 

Minimum 
elongation, 
per cent in 

8 inches 

Minimum 
reduction, 
per cent 
of area 

Plates and shapes.... 

85,000-100,000 

50,000 

l,600,000 l 

ultimate 

40 3 

Eye-bar flats, un- 
unannealed 2 . 

95,000-110,000 

90,000-105,000 

55,000 

55,000 

15 per cent 
1,800,000 

25 

Pins, annealed. 

35 


ultimate 
in 2 inches 


1 For material thicker than one inch (1 in.), the required percentage of elongation shall be 
reduced by one for each increase in thickness of one quarter inch (££ in.) or fraction thereof 
above one inch (1 in.), but in no case shall the minimum elongation required be less than 
14 per cent. 

2 Tests for information shall be made of annealed specimens cut from the rolled eye-bar 
flats. 

3 For material thicker than three-quarter inch (% in.) the required percentage of reduction 
of area shall be reduced by two for each increase in thickness of one-quarter inch in.) or 
fraction thereof above three-quarter inch (% in.) 


Bronze bushings used in heads of hanger eyebars for suspending center 
span and bearing plates with sliding surfaces must meet the following 
physical requirements: 


In tension 

Tensile strength. 

Elastic limit. 

Elongation in 2 inches 
Reduction of area.... 


Pounds per 
Square Inch 

100,000 

60,000 

10 % 

20 % 


In compression 

Elastic limit. 60,000 

Permanent set due to a load of 100,000 pounds per square inch, 0.02. 


177. Bending Tests .—Specimens of nickel steel not less than 2 inches 
wide and of the full thickness of the material as rolled shall bend cold 
180° around rods of the diameters specified below for the various 
thicknesses, without fracture on the outside of the bend. 


For material up to y 2 inch, inclusive. 180° around D = IT 

For material over y inch and up to 1 y inches, 

inclusive. 180° around D = 2T 

For material over iy inches. 180° around D = ST 























ALLOY STEELS 


121 


Angles of all thicknesses shall open cold to an included angle of 150° 
and close to an angle of 30°, without a sign of fracture. 

Steel Castings 

182. Chemical Requirements .—The ladle test of steel for castings shall 
not contain more than the following proportions of the elements named: 


Phosphorus. 0.04 of 1 per cent for basic steel. 

Phosphorus. 0.06 of 1 per cent for acid steel. 

Sulphur. 0.05 of 1 per cent. 

Manganese. 0.75 of 1 per cent. 

Silicon. 0.35 of 1 per cent. 


183. Annealing .—All steel castings shall be carefully and thoroughly 
annealed in a manner approved by the engineer, and shall have a fine¬ 
grained or silky fracture. 

FROM SPECIFICATIONS FOR THE MATERIAL FOR TWO MAIN 
TOWERS OF THE DELAWARE RIVER BRIDGE (1922) 

All structural steel shall be made by the open-hearth process. 

Two kinds of wrought structural steel will be used in the towers, 
designated respectively as carbon steel and silicon steel. 

The various grades of steel shall not contain more than the following 
percentages of elements: 



Carbon steel 

Silicon 

steel 

Mild 

Medium 

Rivet 

Carbon. 

Manganese. 

Phosphorus: 

Acid process. 

Basic process. 

Sulphur. 

Silicon. 

0.06 

0.04 

0.05 

0.06 

0.04 

0.05 

0.04 

0.04 

0.045 

0.40 

1.00 

0.06 
0.04 
0.05 
0.45 | 


The percentage of silicon in silicon steel shall not be less than 0.20. 
Specimens cut from the finished material shall show the following physi¬ 
cal properties: 



























122 


STRUCTURAL ENGINEERING 




Carbon steel 


Silicon steel 


Mild 

Medium 

Rivet 

Tensiles trength, pounds 





per square inch. 

Minimum yield point, 

55,000-65,000 
34 tensile 

62,000-70,000 

52,000-60,000 

80,000-95,000 

pounds per square inch 

strength 

37,000 

30,000 

45,000 

Elongation in 8 inches, 

1,400,000! 

1,500,0001 

1,500,000 

1.600.000 2 

minimum per cent. . . 

tensile strength 

tensile strength 

tensile strength 

tensile strength 

Reduction of area, mini- 





mum per cent 


42 per cent 3 

52 per cent 

35 per cent 4 

Bend test, mat’l. 24 inch 





or less. 

180° flat 

180° around 


180° around 



D = T 

180° flat 

D = T 

mat’l. over YL to 134 

180° around 

180° around 


180° around 

inches. 

D = T 

D = 1.5 T 


D 1.5 = 7' 


D =» Inside diameter of bend. 
T = Thickness of material. 


1 For mild or medium steel material over 24 inch thick, deduct one from percentage of 
elongation for each increase in thickness of 34 inch or fraction thereof, above 24 inch, but 
in no case shall the elongation be less than 4 18 per cent. 

2 For silicon steel, material over 1 inch thick, deduct one from percentage of elongation 
for each increase in thickness of 34 inch or fraction thereof, above 1 inch, but in no case 
shall the elongation be less than 14 per cent. 

3 For medium steel material over 2 4 inch thick, deduct one from percentage of reduction 
of area, for each increase in thickness of 34 inch, or fraction thereof, above 2£ inch, but 
in no case shall the reduction of area be less than 35 per cent. 

4 For silicon steel material over 2£ inch thick, deduct one from percentage of reduction 
of area for each increase in thickness of 34 inch, or fraction thereof, above % inch, but 
in no case shall the reduction of area be less than 24 per cent. 

12. Tests from material of a thickness or diameter in excess of 1 X 
inches, shall show an ultimate strength and yield point equal to the 
minimum specified for its grade and an elongation in 2 inches of 

1,600,000 
tensile strength 

13. All angles shall withstand being opened when tested cold, to an 
angle of 150°, or closed to an angle of 30°, without rupture. 

14. All silicon steel shall be made especially for this work and shall 
be subject to a system of indentification approved by the Engineer and 
shall be handled by itself and isolated in such manner as to prevent the 
possibility of its becoming mixed with other kinds of steel. 

15. All steel shall be of uniform quality of each class. It shall be 
straight, without buckles or kinks, and free from injurious seams, flaws, 
cracks, excessive scale and pitting and other defects. 

16. Every finished piece of structural steel shall have the melt number 
and name of the manufacturer stamped or rolled on it. Shapes and uni¬ 
versal mill plates shall be hot stamped. Sheared plates shall be stamped 




















ALLOY STEELS 


123 


with dies after laying out. Painting melt numbers will not be sufficient. 
Rivet steel and similar small pieces may be shipped in bundles, securely 
tied together, with melt number attached. 

17. Whenever any steel is to be allowed to become cold in any stage 
of rolling, every individual piece shall be distinctively stamped with its 
melt number while still hot. 

18. All cold ingots, blooms or slabs, before being heated preliminary 
to further working, shall have their heat numbers identified and 
approved by the inspector. 

19. The cross-section or weight of each piece of wrought steel shall not 
vary more than 2)^ per cent from that specified, except in the case of 
sheared plates, for which allowance will be made in accordance with the 
specification of structural steel for bridges of the American Society for 
Testing Materials (A7-21), a copy of which is on file in the office of the 
Chief Engineer. 

29. The yield point shall be determined by the drop of the beam of 
the testing machine. The testing machine shall not be stopped to obtain 
the drop of the beam. 

30. All tension fractures shall be silky and of fine texture, free from 
coarse crystals. Square fractures shall be a sufficient cause for rejection. 

31. In case the ultimate strength falls outside of the specified limits 
by less than 1,000 pounds, all other requirements being filled, or in case 
the yield point falls below the specified minimum by less than 1,000 
pounds, all other requirements being filled, then two more tests may be 
taken from material of the same melt and thickness for each test thus 
failing and if both such retests fill all requirements the material will 
be accepted. 

32. The following table is reproduced, by permission, from the 
valuable book by Professor H. F. Moore, on the “Materials of 
Engineering,” McGraw-Hill Book Co., Inc., 1922, and gives a 
summary of average values compiled by him. 


Average V alijes for Strength, Stiffness, and Ductility of Iron and Steel 

The values given in this table are based on test data from various materials testing laboratories 


124 


STRUCTURAL ENGINEERING 


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CHAPTER IX 


NONFERROUS METALS AND ALLOYS 

References: 

Mills, Sec. 6; Johnson, Chap. XXVI; Upton, Chap. XIX. 

1. The structural engineer has little or nothing to do with these 
materials. Some of the brasses (copper-zinc alloys) and some of 
the bronzes (copper-tin alloys) are used in machinery, and are of 
great importance there. Copper, and sometimes aluminum, 
are used for wires carrying electric current, and in the design of 
transmission towers may have to be considered as structural 
materials. 

Only a few brief notes are thus required here regarding these 
materials, and the reader is referred to the following works for 
detailed information (see Chap. I). 

2. Copper is a very ductile and malleable metal with high 
electrical conductivity. Its production is one of the great indus¬ 
tries of the United States, which country produces more than 
one-half of the world’s production. 

The strength of copper depends largely upon the mechanical 
treatment to which it has been subjected. Hot-rolled copper 
plate has an elastic limit of only 7,000 or 8,000 pounds per square 
inch, an ultimate tensile strength of about 33,000, a percentage of 
elongation of about 50: by cold hammering, the elastic limit may 
be raised to above 20,000, the percentage of elongation reduced to 
30, the ultimate strength remaining unchanged. Copper wire, 
according to the specifications of the A.S.T.M., has properties as 
shown by the following table: 


126 


NONFERROUS METALS AND ALLOYS 


127 


Copper Wire 
(A.S.T.M.)' 



Hard-drawn 

Medium hard-drawn 

Soft 

wire 

Diam- 

Minimum 

Minimum 

Tensile strength 

Minimum 

Minimum 

Minimum 

eter, 

tensile 

per cent 



per cent 

tensile 

per cent 

inches 

strength, 

of elonga¬ 

Minimum, 

Maximum, 

of elonga¬ 

strength, 

of elonga¬ 


pounds per 

tion in 

pounds per 

pounds per 

tion in 

pounds per 

tion in 


square inch 

10 inches 

square inch 

square inch 

10 inches 

square inch 

10 inches 

0.460 

49,000 

3.75 

42,000 

49,000 

3.75 

36,000 

35 

0.325 

54,500 

2.40 

45,000 

52,000 

3.00 

36,000 

35 

0.229 

59,000 

1.79 

48,000 

55,000 

2.25 

37,000 

30 



per cent 



per cent 





in 



in 





60 inches 



60 inches 



0.204 

60,100 

1.24 

48,330 

55,330 

1.25 

37,000 

30 

0.102 

64,900 

1.00 

50,330 

57,330 

1.04 

38,500 

25 

0.040 

67,000 

0.85 

53,000 

60,000 

0.88 

38,500 

25 

0.003 






40,000 

20 


These figures show the increase of strength and the decrease of 
ductility as the diameter decreases, that is, as the mechanical 
work put upon the metal increases. The effect of partial anneal¬ 
ing is shown in the figures for medium hard-drawn wire, and that 
of nearly complete annealing in those for soft wire. The elastic 
limit of hard-drawn wire will be 55 to 60 per cent of the ultimate, 
and about 50 per cent for the medium hard-drawn wire. The 
yield point is not well defined. The modulus of elasticity is 
about 16,000,000, varying from about 11,000,000 for cast copper 
to 17,600,000 for hard-drawn copper wire. Copper resists corro¬ 
sion much better than iron or steel, and is sometimes used as a 
roof covering, and also for water conductors, wires for screens, 
etc. The shearing strength of cast copper is about 30,000 pounds 
per square inch. 

3. Zinc is not used in structural work, except as a thin coating 
for iron or steel (galvanizing). The properties of cast zinc, all 
having small (varying) proportions of impurities, are about as 
follows, the figures being averages : 1 

Tensile strength: 3,700 to 11,980 pounds per square inch. 

Compressive strength at 20 per cent deformation in 2.6 inches: 
23,030 to 39,490 pounds per square inch. 

Modulus of rupture: 10,050 to 16,550 pounds per square inch. 

Shearing strength: about 17,000 pounds per square inch. 

Modulus of elasticity: 10,000,000 to 15,000,000 pounds per square 
inch. 

1 See Rigg and Williams, Proc. A.S.T.M., vol. 13, p. 669. 
























128 


STRUCTURAL ENGINEERING 


Some of the variations are due to the impurities, others to the 
physical structure. As in the case of copper, the strength of 
zinc is affected by the mechanical treatment. Zinc is less ductile 
than tin. 

4. Tin is one of the few metals which are not found in large 
quantities in the United States. It is mainly found in Great 
Britain, the East Indies, Australia, and South America. It has 
two allotropic forms, white tin and gray tin. The former is the 
usual form, is very malleable, very resistant to corrosion, and has 
small tensile strength, being but little stronger than lead, or from 
3,000 to 5,000 pounds per square inch. It is used for tin-foil; 
and as sheet tin is used for cans and for roofing. Tin plate is 
sheet iron coated with tin. Gray tin is a dust or powder, and 
white tin when heated moderately tends to change to the gray 
form and become useless, especially when hammered. The 
formation of gray tin is called the “tin disease.” Large quanti¬ 
ties of tin are used, with copper, to form bronzes, which are very 
useful, and possess different properties from either constituent. 

5. Lead. —Lead is easily fusible and is a plastic material, having 
only minor uses in structural engineering; sometimes under bed 
plates to give an even bearing on a rough surface, sometimes 
poured in liquid form as a filling around bolts set into stone. 

6. Aluminum is used widely in the form of tubing and for many 
domestic articles, as well as for wire. It is ductile, very mal¬ 
leable, much hardened by cold working, and very light, but finds 
little application in structures (see Art. 9). Its tensile strength 
varies from about 11,000 pounds per square inch cast to 55,000 
pounds as hard-drawn wire. Its compressive strength is about 
67,000 pounds per square inch. It is sometimes used alloyed with 
copper for parts of automobiles and aeroplanes. 

7. Nickel is a silvery metal, very resistant to corrosion, and 
with a tensile strength ranging from 75,000 pounds per square 
inch to double that value as hard-drawn wire. In structures its 
principal^use is to form an alloy with steel, a small proportion of 
nickel increasing the strength and elastic limit of the steel. 
Almost all the nickel used comes from the Sudbury district of 
Ontario, Canada, and is produced by the International Nickel 
Company, although a small amount comes from New Caledonia. 

8 . The brasses, which are alloys of copper and zinc, and the 
bronzes, which are alloys of copper and tin, are the most important 
nonferrous alloys. There are also alloys of copper, zinc, and tin. 


NONFERROUS METALS AND ALLOYS 


129 


These alloys were exhaustively treated by R. H. Thurston, in the 
third volume of his work on the “Materials of Engineering. 

See also the report of the U. S. Board to test iron, steel, and other 
metals, 1881. 

The addition of zinc increases the strength up to about 45 per 
cent zinc, and the ductility up to about 30 per cent zinc. With 
further additions of zinc, these qualities rapidly diminish, and 
when the percentage of zinc reaches 50 or 60, the alloys are so 
weak and brittle as to be practically worthless. There are many 
brasses. The most usual composition is about two-thirds copper 
and one-third zinc. “Muntz” metal contains 60 per cent copper 
and 40 per cent zinc, and is used for bolts, nuts and other purposes. 
“Manganese bronze’’ (really a brass) contains copper 53 to 62, 
zinc 36 to 45, aluminum 0.05 to 0.5, and lead not over 0.15, with 
small amounts of tin and manganese, and has a tensile strength, 
as cast, of above 70,000 pounds per square inch (A.S.T.M. 
specifications). It is called manganese bronze because a small 
quantity of manganese is added to the melted mass, during 
manufacture, for its deoxidizing effect, although the resultant 
alloy contains little or no manganese. It is very resistant to 
corrosion and to wear (like phosphor bronze). 

The tensile strength of brass reaches 45,000 to 50,000 pounds 
per square inch of original section when the composition is about 
55 to 60 copper and 40 to 45 zinc; and with increasing zinc it 
decreases to about 5,000 for cast zinc. The modulus of elasticity 
of rolled brass is about twelve to thirteen million pounds per 
square inch. Brass is used for hardware, wire, tubes, valves, and 
fittings, and many other purposes, particularly where a non- 

corrosible material is necessary. 

9. The addition of tin to copper, forming bronze, increases 
the hardness and tensile strength up to about 20 per cent tin, and 
the compressive strength up to about 25 per cent, after which the 
strength falls off rapidly. The highest compressive strength is 
about 150,000 pounds per square inch, and the highest tensile 
strength about 35,000 pounds per square inch. The ductility, 
however, falls off rapidly above about 5 per cent tin. The 
properties are considerably affected by heat treatment. Ihe 
modulus of elasticity is between ten and twelve million pounds 
per square inch. The best brasses exceed the best bronzes in 
tensile strength and ductility, but seem inferior in compressive 
strength. One of the strongest bronzes is gun metal, containing 


130 


STRUCTURAL ENGINEERING 


about 90 per cent copper and 10 per cent tin. Bell metal is used 
for bells, on account of its resonance, and contains 20 to 25 per 
cent tin, but is brittle. Machinery bronzes are used for bearings, 
valves, and other machinery parts, and contain 81 to 87 per cent 
copper, many being ternary alloys of copper, tin, and zinc, with 
other elements in some cases. 

Bronze bearing metal for turntables and movable railroad 
bridges consists of copper and tin, with 11 to 20 per cent tin, not 
over about 1 per cent phosphorus, and nearly all the remainder 
copper. 

Phosphor-bronze is bronze which has been deoxidized by 
phosphorus, of which the product may contain very little, and 
which contains 87 to 92 per cent copper, 8 to 10 per cent tin, and 
up to 2.5 per cent zinc. It is used for engine parts, pinions, 
propeller screws, hydraulic cylinders, valve rods, and other parts, 
especially where it is important to resist corrosion, to which it is 
remarkably resistant. It is very hard and very resistant to wear. 

Aluminum and silicon also are deoxidizers, and are used in 
silicon-bronze and aluminum-bronze. The latter contains no 
tin, and, therefore, is not properly a bronze, but is 90 to 95 per 
cent copper and 5 to 10 per cent aluminum. Its strength reaches 
60,000 to 74,000 pounds per square inch with about 10 per cent 
of aluminum. 

The hydraulic engineer is concerned with many metal parts 
which carry a load and are also exposed to friction, such as valves, 
and stems or rods for sluice gates or valves, which are operated by 
screws. In order to make such gates as easy to lift as possible, 
it is desirable to make the diameter of the screws small, so as to 
reduce the frictional moment. The smaller the screw, the 
smaller the frictional moment and the more easily the gate can be 
lifted, if the coefficient of friction remains the same. Such parts, 
too, are often idle for considerable periods, and thus are liable to 
corrode, with resulting increase of the friction. Hence the 
desirability of using a material which does not corrode. 

An interesting and light alloy is duralumin , which has copper 
4.5 per cent, aluminum 94.4 per cent, magnesium 0.6 per cent and 
manganese 0.5 per cent. It has, after rolling, a tensile strength 
of 30,000 to 35,000 pounds per square inch, but by heat treatment 
the tensile strength may be raised to 50,000 to 60,000 pounds 
per square inch, so that it combines great lightness with strength, 
and has been used in the construction of Zeppelins and aeroplanes. 


NONFERROUS METALS AND ALLOYS 


131 


Other light aluminum and copper alloys have been used in auto¬ 
mobiles, such as one containing 92 per cent aluminum and 8 per 
cent copper, which has a tensile strength of 20,000 to 24,000 
pounds per square inch, an elastic limit of 13,000, and weighs 
only about 175 pounds per cubic foot, as against 490 for steel. 1 

10. Ternary alloys, of copper, zinc, and tin are numerous, and 
may be called brasses or bronzes, according as zinc or tin pre¬ 
dominates; or they may be termed alloy brasses or bronzes (U. S. 
Bureau of Standards). 

Professor R. H. Thurston studied the strength of these alloys, 
and proposed a method of representing the results that has been 



widely used in cases where there are three variables. 2 It is 
founded on the principle that in an equilateral triangle ABC , 
(Fig. 37) the sum of the perpendicular distances from any point 
to the three sides is constant. Thus if A, B and C represent 100 
per cent of three variable elements respectively, any point in the 
triangle will have a percentage of A proportional to its perpendic¬ 
ular distance from BC. Thus, dividing each side into tenths and 
drawing lines as shown, point a has 0.30A, 0.20B, and 0.50C. 
Lines drawn on the triangle through points for which the strength 

1 See Cir. No. 76, U. S. Bureau of Standards , on “Aluminum and Its 

^ “The Strongest of the Bronzes,” Trans. A.S.C.E., vol. 10, pp. 1-13, 
1881. 













132 


STRUCTURAL ENGINEERING 


is the same will be contour lines of equal strength. Any function 
studied, such as strength, elongation and yield point, may be 
conceived represented by a surface formed by erecting at each 
point of the triangle a line proportional to the value of the func¬ 
tion when the composition corresponds to the point. This 
method of representation should be fully understood by the 
reader. It is useful in many cases, such as in studying the effects, 
in concrete, of different proportions of fine, coarse, and medium 
sand. 

Professor Thurston found the strongest of the bronzes to 
correspond nearly to copper 55, zinc 43, tin 2, parts in a hundred. 
It had a tensile strength of about 68,900 pounds per square inch 
of original area, or over 92,000 pounds per square inch of frac¬ 
tured area, an elongation of 47 to 51 per cent (length not stated) 
and a reduction of area of about 30 per cent. 

Tobin bronze, which was discovered by J. A. Tobin, of the U. S. 
Naval Engineer Corps, about or before 1875, has copper 58.22, 
zinc 39.48, tin 2.30, as stated by Thurston, and, as cast, has a 
tensile strength of 66,500 pounds per square inch of original sec¬ 
tion, and 71,378 pounds per square inch of fractured section. 
Rolled hot, its tenacity rose to 79,000, and rolled cold to 104,000 
pounds per square inch. Its elastic limit is about 60 per cent of 
the ultimate, and its elongation 15 to 25 per cent, or more, in 8 
inches. It thus has the strength of steel, and is non-corrodible. 
It can be forged or rolled at a low red heat or worked cold, and 
can be bent double either hot or cold. 1 It sometimes contains 
small quantities of lead and iron. 

11. Bearing Metals. —The mechanical engineer is concerned 
with metals to be used in bearings of machinery, such as journal 
bearings. Here the strength and hardness must be sufficient 
to carry the load, and the coefficient of friction should be small. 
A metal which is too hard will be apt to have too large a coefficient 
of friction, and one that is too soft will not have the proper resis¬ 
tance to wear. If the surface is composed of hard and soft parti¬ 
cles, the former will support the load and resist the wear, while 
the latter will be worn down slightly below the hard particles, and 
so will afford opportunity to retain the oil used as a lubricant; 
also allowing the hard particles to be worn to a perfectly smooth 
surface. 


1 Thurston. 


NONFERROUS METALS AND ALLOYS 


133 


A common bearing material is Babbitt metal, which consists of 
tin, copper, antimony and lead. There are many kinds of 
bearing metals on the market, some hard and some soft, for high 
and low pressures respectively. For turntables and movable 
bridges, where the speed of motion is slow and the pressure large, 
the metal used is a bronze, which has been referred to in Art. 9. 
For car journals a softer metal is preferable; such as one having 
about 77 parts copper, 8 tin, and 15 lead. 

12. The weights of the nonferrous metals are about as follows: 


Metal 

Copper. 

. 

Weight per 
Cubic Foot 

. 555 

. 437 

Lead. 

. 710 

Tin . 

.. 456 

Aluminum. 

. 166-168 

Nickel. 

. 518-543 


For “ corrosion cracking” of brass, see Art. 8 of Chap. XIX of 
the volume on “Strength of Materials,” dealing with initial 
stresses. 

For further study of nonferrous alloys consult Mills, Upton, 
Johnson, Moore, and especially Thurston; and also the recent 
work, “Metals and Their Alloys,” by Charles Vickers: London; 
Crosby, Lockwood & Son, 1923. 

Circular No. 101 of the U. S. Bureau of Standards on “Physical 
Properties of Materials” contains much tabulated information. 








CHAPTER X 


STONE 


References: 

Merrill: “Stones for Building and Decoration.” 

Ries: “Building Stones and Clay Products.” 

Eckel: “Building Stones and Clays.” 

Watson: “Engineering Geology.” 

Johnson: Chap. VII. 

Mills: Section 3. 

Watertown Arsenal: Reports for 1894-95. 

1. Stone has been used as a structural material from time 
immemorial, generally in compression, never in pure tension, 
sometimes in flexure, as where a block spans an opening like that 
for a door or window, or the distance between two columns in 
ancient temples. 

2. Kinds of Stone for Structures.—These are: 

Igneous rocks: formed by solidification from a fused condition, 
and crystalline in structure, such as granite. 

Sedimentary rocks: deposited in water, and laminated in 
structure, such as sandstone and limestone. 

Metamorphic rocks: either igneous or sedimentary, which have 
been modified by heat or pressure, generally crystalline and often 
laminated, depending on the pressure, such as gneiss f marble 
and slate. Slate is clay shale which has been consolidated by 
great pressure. 

Stones, even of the same class, differ greatly in strength, hard¬ 
ness, and durability. The strongest and most durable under 
ordinary conditions are the granites and gneisses, but they are 
deficient in fire-resisting power. 

Sandstone consists of sand cemented by some other material. 
Some are hard, dense and strong, while others are soft and weak, 
and will almost crumble in the hand. The same is true of lime¬ 
stones. Before using any stone in important work, unless experi¬ 
ence is available regarding it, tests should be made of its crushing 
strength and durability. 


134 


STONE 


135 


3. Weight and Strength of Stone (see “Tests of Materials 
at Watertown Arsenal/’ 1894-95).—The following are average 
values, but there is great variation between different quarries: 


Strength of Stones 


Kind 

Weight 
per cubic 
foot 
pounds 
(Eckel) 

Ultimate strength, pounds per 
square inch 

Poisson’s ratio 

Compres¬ 

sion 

Shearing 

Modulus 

of 

rupture 
in flexure 

Granite. 

168 

20,000 

2,250 

1,600 

0.172-0.25 

Sandstone. 

157 

12,500 

1,685 

1,450 

0.091-0.333 

Limestone. 

165 

9,000 

1,400 

1,240 

0.27 

Marble. 

175 

12,600 

1,300 

1,500 

0.222-0.345 

Slate 

174 



7,000 









The weights given in the foregoing table are of the stone 
itself. The weight of masonry will be somewhat less, depend¬ 
ing upon the thickness of joints and character of mortar. It 
may be taken about as follows: 


Weight of Masonry 


Kind of stone 

Ashlar 

masonry 

Mortar 

rubble 

masonry 

Dry 

rubble 

masonry 

Granite. 

165 

155 

130 

Sandstone . 

150 

135 

110 

Limestone . 

160 

150 

125 

Marble . 

170 

160 

135 






The variability of stone from different quarries must be borne 
in mind. Thus, granite in compression varies from about 15,000 
to 26,000 pounds per square inch; sandstone from less than 7,000 
to nearly 20,000; limestone from 3,000 or less to over 20,000; 
marble, which is more uniform, from about 10,000 to 16,000. 
For Wisconsin building stone,, the modulus of rupture was found 
to be: for granite, 2,713 to 3,910; for sandstone, from 363 to 1,324; 
for limestone, from 1,164 to 4,659. Merrill gives, for the com¬ 
pressive strength of sandstone at right angles to the natural bed, 
5,481 to 17,500 pounds. The strength parallel to the bed is 
generally, though not always, less than perpendicular to the bed. 




































136 


STRUCTURAL ENGINEERING 


4. Elasticity of Stone. —Stone, like brick and concrete, does not 
deform according to Hooke’s law. The strain does not increase 
proportionally to the stress, but less rapidly, as shown by Fig. 38 
for a granite. The curve is at first concave upward, and then 
nearly straight. This may perhaps be due to the initial com¬ 
pression of the thin layer of plaster which is spread over the ends 
in testing or, if no plaster is used, to the initial crushing of the 
particles at the necessarily rough ends until a full bearing is 
obtained. At all' events, the yielding is proportionally greater 
at first than later, and this initial yielding is mechanical rather 
than elastic. The shape of the curve is just the reverse of those 
for wood and cast iron, for which materials the strain increases 



Proportionate compression 


Fig. 38.—Milford pink granite. (Tests of Metals , 1894.) 


faster than the stress. In stone, the modulus of elasticity increases 
as the load increases. Values given in different books vary, even 
for the same material, because they are not always taken at the 
same point on the curve. If the value is of importance to the 
engineer, he should consult the records of original tests. 

Poisson’s ratio for stone seems to average about 0.25, or about 
the same as for iron and steel, or a little less. 

The strength of stone masonry involves the strength of the 
mortar joints, which are thin, and therefore have greater strength 
than the figures usually given for mortar. Weakness of the 
joints could not cause failure, though it might cause settlement. 










STONE 


137 


If the mortar in the joints disintegrates it may cause failure, 
particularly if the masonry is not laid with coursed joints. 

5. Effect of Shape and Dimensions on Strength. —Experiments 
to determine the difference of strength of rectangular prisms, 
cylinders, and other shapes, and the effect of length, were early 
made by engineers, and are fully described by Bauschinger. 1 
They were generally with small specimens, not over about 1 inch 
square in section. Yicat stated that for geometrically similar 
bodies, similarly loaded, the ultimate strength varied as 
the square of homologous sides, and gave many striking proofs. 
In his experiments he interposed soft cushions of paper between 
his specimens and the machine heads, and Bauschinger found the 
results in some respects entirely different when the end surfaces 
were smoothed and had no layers of soft material. This 
illustrates how one detail in testing may make different experi¬ 
ments impossible of proper comparison. Vicat found that as 
long as the length, l, was less than that of a cubic specimen the 
strength was 


P 


constant , 

—y + constant, 


and Bauschinger found thin specimens tested flat to be very 
strong, as would be expected. It was also found that rectangular 
blocks one over the other without mortar always had consider¬ 
ably smaller strength than single blocks. 

Bauschinger gave the formula 


P 

A 


Varea of sec tion/ nstant constant 
circumference \ 


y/ area\ 

r ) 


but reaches the conclusion that the shape of the section is of very 
minor importance. Hodgkinson reached the same conclusion 
regarding short lengths of cast iron. 

General Gillmore, 2 however, believed that the strength of 
cubes of stone increased as the cube root of the side. He also 
studied the effect of cushions of steel, wood, lead, and leather, the 
results being discordant but generally indicating that the 
strengths were in the proportion of 100,89,65 and 62, respectively. 
A soft cushion, and especially one that can flow, like lead, and 
with the stone surface unpolished, no doubt forces its way into 
the interstices of the stone and diminishes the strength by 
increasing the tendency to split rather than to shear. Unwin 

1 Mittheilungen, vol. 6. 

2 Report of the Chief of Engineers, U. S. A., for 1875. 





138 


STRUCTURAL ENGINEERING 


found the strength between lead plates in one case three-fifths 
and in another three-sevenths of the strength when crushed 
between unyielding surfaces, the ends of the block being 
evened up with plaster. 

6. Effect of Loading Part of the Section.—If, instead of loading 

the entire end of a stone specimen, the load is applied only to a 
part of the area, but still centrally, there is an important effect. 
Bauschinger tested pieces having the shapes shown in Figs. 
396 and c, by which the load was applied to a part of the section 
at one end, either by bevelling the edges or by a steel block, and 
to the entire section at the other end, or to equal parts of the 
section at each end (Fig. 39d). Testing without any soft inter¬ 
mediate layer at the ends, he got no splitting of the blocks longi¬ 
tudinally, but in every case the failure was by the formation of a 



i j 


(a) (b) (c) (d) 

Fig. 39 . 

pyramid having for its base the small pressure area, and an oppos¬ 
ing pyramid not so plainly marked, which wedged off pieces on 
the sides. The strength is shown in the following statement; 
with 4-inch cubes of Swiss sandstone bevelled at one end, if the 
average ratio of small end to large end was 0.182, 0.377, 0.658, 
the average ratio of the ultimate unit stress on the small end to 
that for a complete cube was 1.82, 1.33, 1.13; and the ratio 
of the ultimate unit stress on the large end to that for a 
complete cube was 0.333, 0.51, 0.726. The angle of bevelling 
at the end had little influence. Tests of complete cubes or 
cylinders showed two pyramids, or cones, one at each end; with 
pieces of greater length the length of the pyramids or cones 
increased, and with smaller lengths they decreased, and with 
lengths smaller than lateral dimension the base of the pyramids did 
not occupy the entire face exposed to compression, as it did for 
cubes or longer pieces; and the side pieces or plates split off first, 












STONE 


139 


the hour-glass shaped piece remaining intact, and carrying a 
much greater load. 

Tests with load applied over part of one or both ends (Figs. 396 
and c) gave results as follows for cubes of about 4 inches of 
Swiss sandstone; with steel block at one end only, if the ratio 
of compression area to the full section was 0.154, 0.333, 0.618 
the ratio of ultimate on the small compressed area to that 
for a complete cube was 1.60, 1.40, 1.17 and the ratio of 
ultimate on the large end to that for a complete cube was 
0.247, 0.469, 0.726. With equal steel blocks at each end, if the ratio 
of the compression area to the full section was 0.156, 0.328, 
0.630 the ratio of ultimate on the compression area to that for a 
complete cube was 0.901, 0.749, 0.901 and the ratio of 
ultimate on full cross-section to that for a complete cube was 
0.140,0.247,0.574. These last results are very remarkable: with 
the pressure applied through two equal square blocks of an area 
smaller than the cross-section, the strength of the cube was only 
about as great as for a prism having the area of the blocks 
and the height of the cube, the material outside of this 
prism doing little good. Yicat had already noticed a similar fact. 
It is due to the action of pyramids having bases equal to the area 
of the block, which wedged off side pieces instead of exposing 
them to compression. 

Bauschinger also experimented with eccentric loading applied 
by a steel block on a narrow area parallel to one side and extend¬ 
ing clear across the face. He found the law of planar distribution 
fulfilled in this case. The failure was by the formation of a 
narrow wedge under the bearing area (even when that area was 
close to the side of the cube), which wedged off the sides. He 
gave a formula for an eccentric load applied over a smaller square 
or rectangular area anywhere in the face of the block. 

In testing stone, it is obvious that the pressure should be 
applied over the entire end area. It has been explained that the 
plane on which shearing is greatest is at 45° with the direction 
of the load, but that taking friction into account the plane on 
which shearing would be most likely to occur is steeper than 45°, 
being as high as 60° for brick or stone. If the angle is 45°, it is 
obvious that two pyramids cannot form in a square rectangular 
prism if the piece is shorter than a cube; if 60°, if the height is 
less than 1.73 times the side. With less height than these limits 
the strength should increase. Thus, a brick tested flat should 
give larger strength than if tested on edge or endwise. 


140 


STRUCTURAL ENGINEERING 


In masonry, the function of a mortar joint is to secure even 
distribution of the load and to keep out water. If a bridge bed 
plate rests on stone masonry, a thin layer of cement or a plate of 
lead is generally placed under it. 

7. Allowable Stresses. —The factor of safety for stone is large, 
generally from 10 to 20, or even larger for stone of poor quality. 
It varies with the quality of masonry, being less for the best 
cut-stone masonry, and larger for poorer masonry. For rubble 
masonry, in which the stones are of irregular sizes and shapes, 
and there are no regular joints, the allowable stress may be as 
low as 150 or 200 pounds per square inch. 

The following are figures given by some specifications: 

A.R.E.A., for railway bridges, impact being added: 

Pounds per 
Square Inch 


Bearing on granite masonry. 900 

Bearing on sandstone and limestone masonry. 400 

Bearing on concrete masonry. 600 


Canadian Department of Railways and Canals, for railway bridges, 
impact being added, on first class masonry, and on Portland cement con¬ 
crete, not less than one month old: 

Pounds per 
Square Inch 


Sandstone. 300 

Concrete. 400 

Sound limestone. 400 

Granite. 500 


Massachusetts Public Service Commission, for electric railway and 
highway bridges, impact being added: 

Pounds per 
Square Inch 

(Kind of masonry not specified). 400 

These figures may not be properly comparable, because the percentages 
of impact may not be the same. 

Building Code of National Board of Fire Underwriters: 


Pounds per 
Square Inch 

Rubble stonework in Portland Cement mortar. 140 

Rubble stonework in lime and cement mortar. 100 

Rubble stonework in lime mortar. 70 

Cut stone masonry, other than sandstone. 600 

Sandstone masonry. 300 

Granites, according to test. 1,000 to 2,400 

Gneiss... 1,000 

Limestones, according to test. 700 to 2,300 

Marbles, according to test. 600 to 1,200 

Sandstones, according to test. 400 to 1,600 

Slate. 1,000 





















STONE 


141 


Allowable stresses for flexure or shearing are not generally speci¬ 
fied. If necessary to use them, one-twentieth the ultimate 
stresses given in Art. 3 may generally be safely used, though 
Bauschinger found some Bavarian sandstones for which this 
would be too high. In any important 
case, the stone should be tested. 

Bridge bearings are generally arranged 
as in Fig. 40. 

8. Durability of Stone.—The durability 
of a stone depends upon its composition 
and structure, and the circumstances of 
its exposure. 

As regards composition, some minerals 
are very durable, while others weather 
rapidly. Silicates are generally the most 
resistant, carbonates much less so. Cer¬ 
tain constituents are undesirable. A 
knowledge of mineralogy is necessary in order to judge of this, 
and if the matter is important the advice of a geologist or 
mineralogist should be obtained. 

The most powerful disintegrating agencies are water and 
change of temperature, to which may be added, in certain cases, 
fire and fumes. If stone is likely to be exposed to particular 
fumes, the advice of a chemist should be sought as to whether 
they will have a serious effect. Resistance to fire will be pres¬ 
ently referred to. The most important remaining elements, 
water and temperature, depend mainly on the porosity of the 
stone and the solubility of its constituents in water. All stones 
are more or less porous, and will absorb water. The more porous 
they are, the less desirable. The granites, as a rule, absorb less 
than 1 per cent of their dry weight, many of them less than one- 
half of 1 per cent; sandstones frequently ten times as much; 
limestones and marbles from 1 to 3 or 4 per cent. 1 This absorp¬ 
tion is a good index of the durability and ability to resist the 
action of water and freezing; though something depends upon the 
cohesive strength to resist a disruptive force, for clearly a force 
sufficient to disrupt one stone might not disrupt another. 

From this point of view, the granites and gneisses are the most 
durable, the marbles and limestones next, the sandstones last; 
and experience shows this to be correct as a rule. 

1 See tables in Merrill. 


1 



'//////////////////////////////^ 

//////////////////A, 

1 

fy///////////////A 

I 


Fig. 40. 










142 


STRUCTURAL ENGINEERING 


In judging between two stones, a test of absorption is often 
advisable. It may also be desirable to test the resistance to 
freezing by soaking cubes in water, exposing them to a tempera¬ 
ture well below freezing for a time, and repeating the process a 
number of times; then drying, measuring the loss of weight and 
observing any signs of disintegration. If freezing temperatures 
are not obtainable, Brard’s test may be used, which consists in 
immersing in a solution of sodium sulphate, which is allowed to 
crystallize in the pores. Johnson, however, correctly says that 
“since the results of this test do not bear any fixed relation to the 
results of freezing tests, its value is decidedly questionable.” 

On the whole, experience with a given stone is the best test of 
its durability. 

9. Resistance to Fire.—From this point of view the granites 
are the least resistant, high temperatures causing them to scale 
and crack badly, owing to the different expansion of the different 
constituents. From limestones, the carbonic acid is expelled by 
high heat, and the surface crumbles. Sandstones are generally 
the most resistant, depending upon the cementing material, 
being much better than granites, and generally better than lime¬ 
stones. Brick, however, is better than any of these stones. 

10. Protection of Stone.—Stonework is sometimes coated or 
painted with a coating of preservative, such as paraffine, linseed 
oil, soap and alum solutions, or by Ransome’s process. The 
latter consists in first applying a coat of sodium or potassium 
silicate, and following this, when dry, a coat of calcium chloride, 
producing a silicate of lime. 

11. Specifications.—Specifications for stone masonry often 
prescribe the kind of stone and even the particular quarry. 
Sometimes the choice of several is allowed. Sometimes it is 
merely required that the stones shall be satisfactory to the 
engineer, but this is not sufficiently definite. These methods 
make it unnecessary to specify the strength. It is generally 
required that the stones shall be free from seams or other imper¬ 
fections, and that seasoned stone (that is, stone from which the 
quarry moisture has been allowed to dry out) shall be used wher¬ 
ever there is liability to frost, since freezing might be quite injuri¬ 
ous to unseasoned stone. 

The remaining requirements concern the dimensions and 
arrangement of the stones, the dressing of the surfaces, character 
of joints, etc., and belong under the subject of Masonry. 


CHAPTER XI 


BRICK AND OTHER CLAY PRODUCTS 

1. References: 

Chapters in Mills and in Johnson. 

Searle: “Modern Brick Making.” 

Ries: “Building Stones and Clay Products.” 

2. The Clay products used in structures include brick, hollow 
blocks for building or fireproofing, architectural terra cotta, terra 
cotta lumber, roofing and other tiles, and pipes for drains, sewers 
and culverts. These are all made by molding, pressing, drying, 
and burning clay, the burning often carried to the point of incipi¬ 
ent vitrification. The character of the product is very variable, 
depending upon the raw material, method of manufacture, and 
particularly the degree of burning. 

3. Brick is an important structural material. Well-burned, 
hard bricks should have a generally uniform color, should be free 
from cracks, laminations, or blisters, should ring when struck with 
a hammer, should be so hard that when a brick is broken the 
interior can with difficulty be scratched with a knife, and should 
absorb a small percentage of water. 

4. The A.S.T.M., in Specification C 21-20, gives the following 
classification of building brick: 


Name of grade 

Absorption limits, 
per cent 

Compressive strength 
(on edge), 

pounds per square inch 

Modulus of rupture, 
pounds per square inch 

Mean of 

5 tests 

Individual 

maximum 

Mean of 

5 tests 

Individual 

minimum 

Mean of 

5 tests 

Individual 

minimum 

Vitrified brick.... 

5 or less 

6.0 

5,000 or 

4,000 

1,200 or 

800 




over 


over 


Hard br»ck. 

5 to 12 

15.0 

3,500 or 

2,500 

600 or 

400 




over 


over 


Medium brick.... 

12 to 20 

24.0 

2,000 or 

1,500 

450 or 

300 




over 


over 


Soft brick. 

20 or over 

No limit 

1,000 or 

800 

300 or 

200 




over 


over 



143 



















144 


STRUCTURAL ENGINEERING 


The standing of any set of bricks is to be determined by that 
one of the three requirements in which it is lowest. 

The Committee of the A.S.T.M. instituted an elaborate 
series of tests of bricks. The bricks were tested flatwise in com¬ 
pression, and in flexure flatwise with a span of 6 inches and load 
at the center. Since a brick measures only about 2 by 4 by 8 
inches, there is objection to testing it flatwise in compression, 
and the results will be above the real strength, since the length 
of 2 inches is insufficient to allow a complete shear fracture to be 
developed. More reliable results are obtained by testing them 
edgewise or endwise. Tests by the U. S. Bureau of Standards 1 
showed an average strength when tested flatwise of 6,226, and 
when tested edgewise of 5,399 pounds per square inch. 

Orton states that true vitrification would correspond to an 
absorption of less than 3 per cent, and that if the absorption is 
less than 5 per cent, danger from frost is negligible. 

Softs bricks are unsuitable for use to carry loads and should 
only be used for filling. Structural specifications generally call 
for hard-burned brick. Pressed brick, which have been sub¬ 
jected to heavy pressure in molds after drying and before burning, 
are more nearly perfect in shape than ordinary brick, but are 
costly and only used where appearance is important. 

5. Strength of Brick. Crushing. Mills gives average crushing 
strength about as follows, in pounds per square inch: 

Pounds per 
Square Inch 

Good building brick. 4,000 

Pressed brick.. 8,000 

Sand-lime brick 1 . 3,000 to 4,000 

Paving brick. 10,000 

Fire-clay brick. 3,000 to 6,000 

Terra cotta blocks. 4,000 

Architectural terra cotta. 3,000 

1 Sand-lime bricks are not made from clay, but from a mixture of slaked lime and sand, 
molded and hardened under steam pressure. 

Transverse and Shearing Strength. —The tests at Watertown 
Arsenal indicate, according to Mills, about the following values 
for modulus of rupture and shearing strength: 

1 Proc. A.S.T.M., p. 150, 1915. See also paper by Edward Orton, Jr. 
in vol. XIX, p. 268, 1919. 









BRICK AND OTHER CLAY PRODUCTS 


145 


Kind 

Modulus of 
rupture, pounds 
per square inch 

Shearing 
strength, pounds 
per square inch 

Common building brick. 

500 to 1,000 
600 to 1,200 
300 to 600 
1,500 to 2,500 
300 to 600 
500 to 1,000 

1,000 to 1,500 
800 to 1,200 
500 to 1,000 
1,200 to 1,800 
500 to 1,000 

Pressed brick. 

Sand-lime brick. 

Paving brick. 

Fire-clay brick. 

Unglazed terra cotta blocks. 




6. Elasticity of Brick.—Brick does not follow Hooke’s law, and 
yet tests at Watertown Arsenal indicated a close adherence to 
that law up to loads of 6,000 to 10,000 pounds per square inch. 

































146 


STRUCTURAL ENGINEERING 


Three grades of brick were tested, on end. The strongest aver¬ 
aged 14,000 pounds per square inch and showed an almost con¬ 
stant value of E of 4,100,000 pounds per square inch up to 10,000 
pounds per square inch; the next grade averaged 10,500, and 
showed, up to 6,000, E nearly constant and equal to 3,400,000; 
the third grade averaged 7,500, and showed E equal to 2,000,000 
up to 6,000. 1 Softer grades would no doubt show smaller 
values of E, and diagrams convex upward, like those for concrete. 
Figure 41 shows the stress-strain diagram. 

7. Weight.—The specific gravity of brick is given by Johnson 
as from 1.9 to 2.6, according to the character of the raw materials 
and the degree of burning. This corresponds to a weight per 
cubic foot of 118 to 162 pounds. 

The weight of brickwork will depend also upon the character 
of mortar and thickness of joints, as in other masonry, and may 
be taken as from 100 to 140 pounds per cubic foot. 

8. Strength of Brick Piers.—This will depend not so much on 
the strength of the bricks alone, as upon the bond (arrangement 
of bricks in the courses), the character of the mortar and thickness 
of joints, and the workmanship. A pier generally will fail by 
breaking up into smaller columns by transverse fracture of 
individual bricks. This will depend also upon the size of the 
column and the character of bond possible, and the regularity 
in form of the brick, which will affect the tendency to break when 
laid up in a pier. A building brick measures approximately 8 
by 4 by 2 inches, and is laid flat. If the side (8 by 2 inches) shows 
on the face, the brick is called a stretcher; if the end (4 by 2 inches) 
shows, it is a header. If all faces showed stretchers only (except 
the end bricks) the pier would consist of an outer shell 4 inches 
thick, not tied to the inside, and a load on this shell would not be 
distributed. There should be a certain proportion of headers. 
In common bond, all are stretchers for a certain number of 
courses, and then there is a course entirely of headers. In 
English bond courses of stretchers and headers alternate. In 
Flemish bond each course has alternate headers and stretchers. 

The strength of brick piers depends upon the character and 
strength of the brick, the method of bonding, the character and 
strength of the mortar, the age, the dimensions, and the work¬ 
manship. The number of variables explains the divergent 
results. Technologic Paper 111 of the Bureau of Standards gives 

1 “Tests of Metals,” p. 1138, 1885. 


Number 


BRICK AND OTHER CLAY PRODUCTS 147 


Tests of Brick Piers, T.P. Ill U .S. Bureau of Standards 


Piers 


Brick 


1 

2 

3 

4 

5 

6 

7 

8 
9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 
21 
22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

44 

45 

46 

47 

48 

49 

50 


District 

Grade 

Bond 

Mortar 

Height, ft. 

Area,sq.in. 

Age, days 

Max. unit load 

lbs. per sq. in. 

fe 

lbs. per sq. in. 

U 

lbs persq.in. 

fb 

lbs. per sq. in. 

Per cent 

absorption 

P 

1 

1:1 

A 

10 

930 

30 

2,710 

11,990 

8,900 

1,945 

4.08 

P 

1 

1:3 

A 

10 

930 

30 

2,740 

11,990 

8,900 

1,945 

4.08 

P 

1 

1:6 

A 

10 

930 

30 

2,900 

11,990 

8,900 

1,945 

4.08 

P 

2 

1:1 

A 

10 

856 

32 

2,000 

7,880 

6,450 

1,370 

7.46 

P 

2 

1:3 

A 

10 

885 

32 

2,070 

7,880 

6,450 

1,370 

7.46 

P 

2 

1:6 

A 

10 

946 

33 

870 

2,450 

2,040 

675 

15.16 

P 

3 

1:1 

A 

10 

1,024 

29 

510 

1,659 

1,350 

345 

16.28 

P 

3 

1:3 

A 

10 

1,043 

29 

560 

1,659 

1,350 

345 

16.28 

P 

3 

1:6 

A 

10 

1,024 

32 

650 

1,659 

1,350 

345 

16.28 

P 

1 

1:1 

C 

10 

841 

90 

3,800 

11,965 

10,050 

2,775 

1.28 

P 

1 

1:3 

C 

10 

841 

90 

3,220 

11,965 

10,050 

2,775 

1.28 

P 

1 

1:6 

c 

10 

841 

90 

3,300 

11,965 

10,050 

2,775 

1.28 

P 

2 

1:1 

c 

10 

908 

33 

1,760 

7,880 

6,450 

1,370 

7.46 

P 

2 

1:3 

c 

10 

961 

32 

870 

2,450 

2,040 

675 

15.16 

P 

2 

1:6 

c 

10 

878 

30 

1,760 

7,880 

6,450 

1,370 

7.46 

P 

1 

1:1 

B 

10 

940 

120 

1,450 

11,990 

8,900 

1,945 

4.08 

P 

1 

1:3 

B 

10 

940 

120 

1,270 

11,990 

8,900 

1,945 

4.08 

P 

1 

1:6 

B 

10 

940 

120 

1,360 

11,990 

8,900 

1,945 

4.08 

P 

2 

1:1 

B 

10 

906 

120 

840 

7,880 

6,450 

1,370 

7.46 

P 

2 

1:3 

B 

10 

906 

120 

890 

7,880 

6,450 

1,370 

7.46 

P 

2 

1:6 

B 

10 

900 

120 

990 

7,880 

6,450 

1,370 

7.46 

P 

3 

1:1 

1 lime; 3 sand 

10 

1,024 

120 

210 

1,659 

1,350 

345 

16.28 

P 

3 

1:3 

1 lime; 3 sand 

10 

1,024 

120 

178 

1,659 

1,350 

345 

16.28 

P 

3 

1:6 

1 lime; 3 sand 

10 

1,024 

120 

126 

1,659 

1,350 

345 

16.28 

0 

1 

1:1 

C 

10 

841 

29 

1,450 

7,340 

4,910 

733 

16.80 

0 

1 

1:3 

C 

10 

841 

29 

1,760 

7,340 

4,910 

733 

16.80 

0 

2 

1:1 

C 

10 

841 

31 

1,630 

6,880 

5,490 

893 

16.40 

0 

2 

1:3 

C 

10 

841 

30 

1,790 

6,880 

5,490 

893 

16.40 

0 

3 

1:1 

C 

10 

841 

30 

1,880 

6,510 

5,700 

1,090 

17.10 

0 

3 

1:3 

C 

10 

841 

29 

1,690 

6,510 

5,700 

1,090 

17.10 

0 

3 

1:6 

C 

10 

841 

31 

1,660 

6,510 

5,700 

1,090 

17.10 

Y 

1 

1:1 

c 

10 

791 

32 

1,170 

5,630 

6,440 

601 

16.40 

Y 

1 

1:3 

c 

10 

791 

32 

1,300 

5,630 

6,440 

601 

16.40 

Y 

1 

1:6 

c 

10 

791 

31 

1,260 

5,630 

6,440 

601 

16.40 

Y 

2 

1:1 

c 

10 

784 

30 

1,280 

4,430 

5,449 

616 

18.60 

Y 

2 

1:3 

c 

10 

791 

31 

1,280 

4,430 

5,449 

616 

18.60 

Y 

2 

1:6 

c 

10 

791 

31 

1,220 

4,430 

5,449 

616 

18.60 

Y 

3 

1:1 

c 

10 

791 

31 

1,070 

2,710 

2,970 

497 

19.30 

Y 

3 

1:3 

c 

10 

791 

31 

1,060 

2,710 

2,970 

497 

19.30 

Y 

3 

1:6 

c 

10 

791 

30 

1,020 

2,710 

2,970 

497 

19.30 

c 

1 

1:1 

c 

10 

841 

32 

840 

3,200 

3,010 

1,180 

16.20 

c 

1 

1:3 

c 

10 

812 

29 

790 

3,200 

3,010 

1,180 

16.20 

c 

1 

1:6 

c 

10 

812 

30 

810 

3,200 

3,010 

1,180 

16.20 

c 

2 

1:1 

c 

10 

812 

30 

750 

3,150 

2,710 

1,140 

16.20 

c 

2 

1:3 

c 

10 

812 

30 

70C 

3,150 

2,710 

1,140 

16.20 

c 

2 

1:6 

c 

10 

812 

30 

71C 

3,150 

2,710 

1,140 

16.20 

p 

2 

1:1 

c 

5 

915 

3C 

1,73C 

2,450 

2,040 

675 

15.16 

p 

2 

1:1 

c 

5 

915 

3C 

1,84C 

2,450 

2,040 

675 

15.16 

p 

2 

l:li 

c 

5 

924 

3C 

2,27C 

2,450 

2.04C 

675 

15.16 

p 

2 

1:1* 

c 

5 

930 

3C 

1.47C 

2,450 

2,04C 

675 

15.16 


1 Galvanized iron wire mesh in every joint. 

2 Galvanized iron wire mesh in every fourth joint. 













































148 


STRUCTURAL ENGINEERING 


a review of previous work, and the results of 50 tests of piers 
about 30 by 30 inches by 10 feet, with a thin cushion of plaster of 
Paris to even up the ends, with different mortars and bond, and 
three grades of brick from different parts of the country. 

Grade 1 was “hard-burned or best quality.” 

Grade 2 was “medium-burned or considered as common.” 

Grade 3 was “ soft-burned or poorest product marketed.” 

The mortars used were: 

A: 1 part Portland cement to 3 parts sand. 

B: 1 part hydrated lime to 6 parts sand. 

C : 1 part composed of 15 per cent hydrated lime (dry weight) and 85 
per cent Portland cement (dry weight) to 3 parts sand by weight. 

The table on page 147 gives the results of the tests; P meaning 
brick from the Pittsburgh district, 0 the New Orleans district, Y 
the New York district and C the Chicago district. The three col¬ 
umns before the last give f c = average compressive strength of the 
bricks flat, fj on edge, /*> the average modulus of rupture. The age 
is in days. The ratios of bond are header to stretcher courses; thus 
1:3 is one header course to 3 stretcher courses. A study of these 
tests is illuminating; thus the low strength of No. 14 is explained 
by the low strength of the bricks and the large percentage of 
absorption; clearly this figure should not be averaged with those 
just above and below as the strength of Grade 2 piers (though 
this is done in some abstracts of these tests). 

It was found from these tests that there was an approximately 
linear relation between the strength of the piers and the trans¬ 
verse and the compressive strength (flat or on edge) of the brick, 
expressed by the relation 

max. unit strength of pier (per square inch) = kf 
where / is either f c , //, or f b . The values of k are given in the 
following table: 


! 


BRICK AND OTHER CLAY PRODUCTS 


149 


Relation between Strength of Brick Pier and Transverse and 
Compressive Strength of Brick 


I 

Kind of stress 

How tested 

Mortar 

k 

Transverse. 

Flat, 7-inch span 
Flat, 7-inch span 
Flat, 7-inch span 
Flat 

A 

1.45 

Transverse. 

Transverse.. 

C 

B 

1.25 

0.65 

Compression. 

A 

0.27 

Compression. 

Flat 

C 

0.26 

Compression . 

Flat 

B 

0.11 

Compression . 

On edge 

On edge 

On edge 

A 

0.32 

Compression. 

C 

0.30 

Compression. 

B 

0.14 





The modulus of elasticity varied approximately as the strength 
of the pier, for cement and cement-lime mortars. Its maximum 
value was 3,500,000 for tests 10 and 12, and its minimum 270,000 
for test 14; the lowest for mortar A was 533,000 for test 9, and for 
mortar C 484,000 for test 38. 

The primary failure was found to arise from transverse failure 
of individual bricks; this shows the importance of thin joints of 
uniform thickness, and uniform bedding of the bricks. Porous 
bricks must be well wet down. The tests show that 15 per cent 
of cement (by weight, or about 35 per cent by volume) may 
be replaced by hydrated lime without lessening the strength, 
and in some cases increasing it. This is probably due to the 
fact that the mortar of cement-lime is easier working than mortar 
of pure cement, and that better bedding is the result. There is a 
slight increase of strength if wire mesh is used, preferably in every 
joint. 

In Bull No. 27 of the University of Illinois, tests are described 
of 16 brick piers and 16 piers of terra cotta blocks. Of the brick 
piers, 14 were of shale building brick, a high-grade, hard-burned 
material with under 3 per cent absorption; and 2 of under-burned 
clay brick with about 17 per cent absorption, softer than would 
be used in building construction; the two thus representing 
extremes in quality. The following table gives a summary of 
the results: 


















150 


STRUCTURAL ENGINEERING 


Brick Piers 

Bulletin No. 27, University of Illinois, 1908 





Average ulti¬ 

Ratio of 


Characteristics of columns, 


mate strength; 

strength of 

Brick 

12H by 12>£ inches by 10 feet 

Age 

usually average 

column to 


approximately 


of 2 tests pounds 

strength of 


| 


per square inch 

brick flatwise 



Well-laid; 1 P.C.; 3 sand 

67 days 

3,365 

0.31 



Well-laid; 1 P.C.; 3 sand 

6 months 

3,950* 

0.37 

Hard-burned < 


Well-laid; 1 P.C.; 3 sand 1 
load 1 inch eccentric / 

68 days 

2,800 2 

0.26 

shale 


Poorly-laid; 1 P.C.; 3 sand 

69 days 

2,920 

0.27 



Well-laid; 1 P.C.; 5 sand 

65 days 

2,225 

0.21 



Well-laid; 1 Nat. cem; 3 sand 

66 days 

1,750 

0.52 



Well-laid 1 lime; 2 sand 

66 days 

1,450 

0.14 

Underburned 

clay 

Well-laid 1 P.C.; 3 sand 

63 days 

1,060 

0.31 

• 


1 In one of these columns the load was repeated seven times before failure occurred. 

2 Computed stress at inside is 4,150 pounds per square inch. 


Engineering Paper No. 12 of Columbia University, Apr., 1923, 
by A. H. Beyer and W. J. Krefeld, contains an account of an 
extensive series of tests of brick piers, 135 piers in all, all 12 inches 
square, 115 being 40 inches high and 20 being 84 inches high. 
All variables were kept constant except the kind of brick, the 
mortar being 1:3 Portland cement. Clay brick was used in 44 
piers, concrete brick in 79, sand-lime brick in 8, and monolithic 
concrete in 4, all in common use in New York City. As in the 
Bureau of Standards tests, the ratio k of ultimate compressive 
strength of pier to ultimate compressive strength of clay bricks 
appeared to be nearly constant, but different in different series of 
tests in which the modulus of rupture of the brick and the 
strength of the mortar differed. For concrete brick the ratio k 
decreased slightly as the compressive strength of the brick 
increased. The concrete brick piers generally failed by diagonal 
shear. The reader should consult this interesting paper. 

The reader should also consult the Watertown tests. They 
are described in the above-mentioned Bull. No. 27. 

9. Allowable Load on Brickwork. —No generally applicable 
figure for this can be given, as so much depends upon conditions. 
The following are given in the Building Code recommended by 
the National Board of Fire Underwiters for hard-burned brick 
with an average compressive strength of 3,000 pounds per square 
inch tested flatwise: 













BRICK AND OTHER CLAY PRODUCTS 


151 


Pounds per 
Square Inch 


Brickwork in portland cement mortar. 250 

Brickwork in natural cement mortar. 208 

Brickwork in lime and portland cement mortar. 208 

Brickwork in lime mortar. Ill 


Cement and cement-lime mortar is to be not weaker than 1:3; 
lime mortar not weaker than 1:4. 

Terra Cotta .—Terra Cotta is made like brick and in a great 
variety of forms. Hollow blocks are used for floor arches, parti¬ 
tions, walls, columns, fireproofing, etc. Terra cotta lumber is 




Fig. 42. (National Fire-Proofing Co.) 



made from clays mixed with finely cut straw or sawdust. The 
straw or sawdust is consumed in burning, leaving a light and 
porous material into which nails and screws may easily be driven, 
and which can be cut with a saw. Hollow blocks and fireproofing 
are made from clay without sawdust, and resemble brick. The 
strength of these products is very variable. Tests of hollow 
blocks, loaded on end, described in Bull. No. 27, University of 
Illinois gave 3,350 to 9,070 pounds per square inch of net section, 



































152 


STRUCTURAL ENGINEERING 


averaging 5,450. These blocks measured about 4 by 8^ by 
8 inches. The strength of columns 10 or 12 feet high varied from 
2,710 to above 3,800 pounds per square inch of gross section. 

Figures 42 and 43 give some forms of terra cotta blocks. 
The reader should consult Sweets’ Engineering Catalogue and 



Fig. 43. 


Sweets’ Architectural Catalogue, very valuable works, for 
advertisements and cuts by the various companies which manu¬ 
facture clay products. These products are becoming more and 
more useful each year. 






CHAPTER XII 


CALCAREOUS CEMENT AND CONCRETE 1 

1. A cement is a material which in a plastic, amorphous, or 
liquid form, fills the voids between a mass of particles, or is 
placed between and coating two surfaces, and which, by after¬ 
wards hardening, binds those particles or surfaces together. It 
does this either by virtue of its adhesion to the surfaces—as 
where it is used to cement together two pieces of glass or crockery 
or wood—or also by virtue of the fact that it coats and surrounds 
and fills the voids between a mass of particles like a mass of sand 
or gravel, so that the mass cannot be separated, or any particle 
dislodged (except a particle on the very surface) without breaking 
the hardened cement. We have seen how the theory of amor¬ 
phous material in steel assumes that the voids between the 
crystals are filled with this material, which acts like a cement, 
though it is of the same character as the material of the crystals 
themselves. Generally, however, cements are very different in 
character from the particles they serve to bind together. 

A cement may harden in several ways. It may harden with 
water by forming crystals out of its own constituents; in this case 
there will be voids between these crystals, which should, for the 
best results, be filled with some amorphous ( i.e ., uncrystalline) 
material, like the amorphous material in steel. It may harden 
by absorbing constituents from the air or from the materials it 
surrounds, and so form crystals or amorphous material. Or, it 
may harden without crystallizing, as glue or glass harden, remain¬ 
ing amorphous or colloidal. (A colloid is a non-crystalline or 
amorphous substance, often or generally glue-like or gelatinous, 

1 For an excellent and clear introduction to the subject of Cement, the 
reader is advised to read Chap. XX of Upton’s “ Materials of Construction.” 
The writer knows of no better introduction to the study of the more extended 
treatises than this. In addition to the other treatises that have been 
referred to, the reader should consult Eckel, “ Cements, Limes, and Plasters,” 
2d edition, 1922, for an excellent treatment of this whole subject. 

One of the very best works dealing with this subject is “A Treatise on 
Concrete, Plain and Reinforced” by F. W. Taylor and S. E. Thompson, 
published by John Wiley & Sons, Inc. 

153 


154 


STRUCTURAL ENGINEERING 


which, however, may harden and become quite strong, its par¬ 
ticles being very finely divided. Some writers on cements mention 
amorphous material, while others mention colloidal material, 
meaning the same thing.) 

There are many kinds of cement, used for uniting many kinds 
of material. Cements used in engineering are almost universally 
calcareous, that is, they have lime as a main constituent; and this 
chapter deals only with these. They are produced from some 
form of limestone, including chalk and marl. 

2. Limestone, if pure, is carbonate of lime (CaC0 3 ). No 
limestones are pure, but all contain foreign ingredients, sometimes 
iron compounds, and often magnesium. Magnesia, belonging 
to the same chemical class as lime, acts like it in many ways, as 
in cements, but is inferior and in some respects injurious, as will 
be seen. 

By heating limestone, the C0 2 is driven off and CaO, or 
quicklime, remains. It comes from the kiln in lumps, which 
are crushed or ground. Quicklime has the property of slaking , 
or uniting with water to form hydrated lime (CaO, H 2 0), which it 
does with great evolution of heat, and expansion to more than 
double its former volume. Magnesian quicklimes (as they are 
called) slake much more slowly than calcium quicklime—so 
slowly that if the quicklime contains much magnesium it may not 
slake properly before using. This is one of the sources of danger 
when cement contains much magnesia, as will be seen. Quick¬ 
limes are purposely slaked before being used for mortar, 
sometimes where the construction work is being done, sometimes 
at the factory before being shipped, the latter product 
being known specifically as “hydrated lime.’ 7 It is a dry powder, 
and is the same as quicklime slaked on the work, except that when 
slaked on the work there is danger, through carelessness or 
incompetent supervision, of burning or of incomplete hydration, 
which dangers should not exist when hydrated lime is prepared as 
a manufactured product. “Hydrated lime” should therefore be 
superior to ordinary slaked quicklime, and it is much used. 

3. Quicklime, on slaking with water, forms either crystalline or 
amorphous calcium hydroxide, depending mainly upon the fineness 
and the degree of burning. Crystalline material forms when the 
hydration is slow, amorphous material when it is rapid. The finer 
the lime, the more rapid the hydration, and the greater the propor¬ 
tion of amorphous material. The lower the burning temperature, 


CALCAREOUS CEMENT AND CONCRETE 


155 


the more rapid the hydration, and the greater the proportion of 
amorphous material. The less water used in mixing, the less 
crystalline material, since the crystals form from solution. 
Quicklime merely exposed to the air will slake, by absorbing 
moisture, to the amorphous condition, the expansion in slaking 
causing the lumps to become powder. 

Lime hydrate, in the presence of moisture, combines with C0 2 
from the air, liberating water which evaporates, and becomes 
limestone, from which it came originally, thus completing the 
cycle. The hardening of lime mortar, therefore, consists first in 
the hydration to a more or less hard hydrate, and afterward the 
slow conversion of this hydrate to limestone. The interior of 
a masonry joint of lime mortar may never become limestone, for 
the penetration of the C0 2 is slow. In the conversion to 
carbonate, a large contraction occurs, so that large cracks would 
be formed. To prevent this and also for economy, lime mortar is 
made by adding a considerable proportion of sand to the paste of 
hydrated lime. 

4. Mortar from lime slaked on the work is made by adding 
sand to the wet plastic hydrate. Mortar made from hydrated 
lime is made by mixing the sand with the dry hydrate powder, 
and then adding water; in this way a more homogeneous mixture 
is obtained. The mortar from hydrated lime may be made 
immediately; while quicklime slaked on the work must be allowed 
to season for at least a day and often much longer, to secure 
complete hydration. Hydrated lime is more crystalline than 
ordinary slaked lime, and hence will not bear so large a proportion 
of sand as the latter; it gives a less plastic mortar, which does not 
work as smoothly as that from ordinary slaked lime. Being 
more crystalline, however, the mortars from “hydrated lime” are 
stronger, shrink less, and set more quickly. 

Lime mortars are much used for ordinary buildings, where 
much strength is not necessary. 

Lime mortar will not set under water, for the hydrate is soluble 
in water, and would be washed out. It is, therefore, not a 
hydraulic material. 

5. Strength of Lime Mortars.—The strength of lime mortar 
is very variable. The greater the proportion of sand, the less 
the strength, as a rule. Fine sand makes a stronger mortar than 
coarse sand, and magnesium limes make a stronger mortar than 
calcium limes. The strength increases with age. It decreases 


15G 


STRUCTURAL ENGINEERNG 


with the amount of water used in mixing. Mills (pages 1, 21) 
gives a table of tests which may be referred to. The compres¬ 
sive strength may vary from 100 to 400 or 500 pounds per square 
inch. The Committee of the A.S.T.M. proposes a specification 
requiring the tensile strength of a mortar of one part by weight 
of lime and three parts by weight of standard Ottawa sand (see 
Art. 15) to be not less than 15 pounds per square inch after 7 
days in air. 

The compressive strength of lime mortar, tested in cubes, even 
with one part of lime to four of sand, should not be less than 100 
pounds per square inch, and may be much greater. A thin 
joint of mortar would have much greater strength than a cube, 
and if the mortar has a strength of 100 pounds per square inch 
tested as a cube, a joint 9 by inches, in which the ratio of height 
to width is 18, may have a strength of 1,800 pounds per square 
inch, and so be able to support the weight of a quite high wall. 

6. Cement.—Lime is not “cement,” though it is “a cement.” 
Calcareous cementing materials are classified as common lime, 
hydraulic lime, natural cement, Portland cement, and slag or 
puzzolana cement. A hydraulic material is one which will set 
under water. 

Common limes have just been described. They are not 
hydraulic. 

It was long ago found that by mixing common lime with a 
certain amount of argillaceous material, a product would result 
which, with proper manufacture, would have hydraulic proper¬ 
ties. This is due to the fact that, if intimately mixed and burned, 
certain calcium silicates and calcium aluminates are formed, 
which when mixed with water, will set and harden in air or under 
water. There are no such compounds in ordinary lime mortar, 
because there is no silica or alumina. These silicates and alumi¬ 
nates hydrate with water by taking up water and forming crystal¬ 
line hydrates, with some amorphous hydrates, which set quickly 
compared with lime, and are insoluble in water, and so are able to 
set under water. To the extent to which the mixture contains 
more lime than combines with the silica and alumina (and iron 
if that element is present) 1 there is free lime , which hydrates 
as already explained. The differences in hydraulic limes and 
cements arise, therefore, from the differences in character and 

1 Newberry states that “iron oxide combines with lime in the same 
manner as alumina.” 


CALCAREOUS CEMENT AND CONCRETE 


157 


amount of the argillaceous components, and the degree of 
burning. 

Hydraulic limes are made by burning slightly argillaceous 
limestones at a low temperature. They will slake slowly, and 
have feeble hydraulic properties. They are little used, as they 
are weak, and set slowly. 

Natural cements are made by burning distinctly argillaceous 
limestones at a comparatively high temperature, sufficient to 
cause the complete combination of lime with silica and alumina 
into silicates and aluminates. They may have free lime, and if so 
this will slake, but the cement as a whole cannot be said to slake. 
They will set under water. 

Puzzolana or Slag Cements are made by mixing slaked lime with 
granulated blast-furnace slag, or a natural volcanic ash called 
puzzolana, without subsequent burning. The slag or puzzolana 
are silicious or argillaceous. The product will not slake, and has 
hydraulic properties. It must not be confused with the product 
obtained by mixing hydrated lime with blast-furnace slag, and 
then burning and grinding, which is a true Portland cement. 

German “Iron Portland Cement’' is a mixture of 70 per cent 
Portland Cement with 30 per cent of granulated blast-furnace 
slag. 

Portland cement is what is now generally understood when 
hydraulic cement is referred to, for its manufacture has increased 
greatly and its strength and other good qualities render it the 
most satisfactory calcareous cementing material. The rest of this 
chapter refers to Portland cement. It is defined as the product 
obtained by finely pulverizing the clinker produced by burning 
to incipient fusion an intimate and properly proportioned arti¬ 
ficial mixture of argillaceous and calcareous materials, with no 
additions subsequent to calcination excepting water and calcined 
or uncalcined gypsum. Clinker is the material as it comes, in 
lumps, from the furnace. The main difference between natural 
and Portland cement is that the former is obtained by burning a 
natural rock and the latter by burning an artificial mixture. If 
the mixtures were the same, and of the same homogeneity, in 
the two cases, the two would be the same, if burned at the same 
temperature and ground to the same fineness. The calcareous 
materials used may be pure limestone, chalk, or marl, or hydrated 
lime. The argillaceous materials may be clay, slate, or shale, 
impure limestones containing clay, or blast-furnace slag. Know- 


158 


STRUCTURAL ENGINEERING 


ing the chemical composition of the raw materials, they may be 
mixed in proper proportion to produce the silicates or aluminates 
required in the cement, leaving little or no free lime or free clay. 
The raw materials are crushed, ground, mixed in proper propor¬ 
tion, and then burned in a revolving kiln. The clinker comes out 
in lumps, and is then ground into cement. The silicates and 
aluminates would set too fast for practical purposes, and so plaster 
of Paris or gypsum is added to retard the set. Gypsum is a 
sulphate, which is injurious if in too large quantity, so that the 
amount of retarder added is limited. 

7. A typical average analysis of Portland cement would be 
approximately, taking a well-known brand as typical: 


510 2 .. 
AI2O3. 

Fe 2 0 3 

CaO.. 

MgO. 

50.. . 


Per Cent 
. 21.08 
. 7.86 

. 2.48 

. 63.68 
. 2.62 
1.25 


When mixed with a suitable amount of water, such a cement 
becomes hard, or sets, in a few minutes, and afterward continues 
to harden for a long time. 1 This is the result of the hydration 
and crystallization of the silicates and aluminates, and the 
formation of some amorphous material which gradually hardens 
as glue or melted glass hardens, and which may slowly crystallize. 

8. The important physical properties of cement, for the engi¬ 
neer, are specific gravity, fineness, time of setting, soundness, and 
strength. 

9. Specific Gravity.—This quality was formerly considered 
useful as a means of detecting adulteration and underburning, 
since the specific gravity of underburned cements is slightly less 
than that of hard-burned cements. The difference, however, is 
very small, and not of importance. The test may detect adulter- 

1 The setting and hardening of hydraulic cements has been carefully 
studied by chemists. The reader may pursue the subject in the following 
readily accessible sources: 

1. Taylor and Thompson: Chap. VI, by S. B. Newberry. 

2. Klein and Phillips: “Hydration of Portland Cement”; Technologic 
Paper 43, U. S. Bureau of Standards, 1914. 

3. Bates and Klein: “Properties of Calcium Silicates and Calcium 
Aluminate Occurring in Normal Portland Cement,” Technologic Paper 78, 
U. S. Bureau of Standards, 1917. 








CALCAREOUS CEMENT AND CONCRETE 


159 


ation, if considerable and with a substance whose specific gravity 
is different from that of the cement; but the other tests would in 
such case probably show unsuitable material. A reduction of 
specific gravity may be due to seasoning, or hydration of free 
lime and absorption of C0 2 by exposure to the air; but hydration 
of free lime previous to use is desirable. The importance of 
specific gravity is therefore small. Standard specifications pro¬ 
vide that the specific gravity shall not be less than 3.10, but 
that the test will not be made unless specifically ordered. The 
test is made by dropping a given weight of cement into a glass 
flask filled with kerosene or benzine up to a given mark on its 
long graduated stem, and noting the rise of the liquid in the 
stem. This gives the volume of liquid displaced, or the volume 
of a given weight of cement, from which the specific gravity is 
found. 

10. Fineness. —The standard A.S.T.M. specification requires 
that the residue on a No. 200 sieve shall not exceed 22 per cent 
by weight. A No. 200 sieve is one having 200 wires per inch 
(or between 192 and 208), the diameter of wire being 0.0021 inch 
(or between 0.0019 and 0.0023), so that the average opening 
between wires is 0.0029 inch. The openings will not be uniform, 
but must not exceed 0.005 inch. 

It was long ago shown that the coarse particles of cement are 
inert, and scarcely more effective than so much sand; notably by 
Eliot C. Clarke in his experiments made while building the main 
drainage works of Boston, 1 and by many experiments since. 
The impalpable flour is the most effective cementing material, 
but the exact size at which a particle becomes inert is not known. 
This is because the coarser particles are not easily accessible to 
water throughout, and so do not easily crystallize into the 
hydrated silicates and aluminates. The outside of coarse partic¬ 
les may be partly vitrified, but even if not, the chemical action 
on the outside seals, as it were, the inside, and prevents or retards 
action there. Obviously, then, the strength of a mortar of a 
given cement will increase with the fineness, though different 
cements of the same fineness will not show the same strength. 
Cements of average fineness will be well within the prescribed 
limits, and will leave a residue of only 10 or 15 per cent, while 
some leave but 6 per cent, and very fine cements may leave but 

1 See Trans. A.S.C.E., vol. XIV, p. 141, 1885. See also Duff A. Abrams, 
A.S.T.M ., 1918. 


160 


STRUCTURAL ENGINEERING 


2 per cent. Fine grinding, however, is expensive, and it is a 
question what standard of fineness is most economical. 

Fine grinding promotes soundness (see Art. 12) by favoring 
complete and rapid hydration. For the same reason, it makes a 
cement more quick setting. Since the coarse particles are inert, 
it also increases the sand-carrying capacity of a cement; in other 
words, the greater the fineness, the greater the proportion of sand 
that may be mixed with the cement to produce a mortar of given 
strength, or the greater the strength of a mortar with a given pro¬ 
portion of sand. 1 Fine grinding is more effective in increasing 
the strength at 7 days than at 28 days or longer periods. It 
lowers the weight per cubic foot about % pound for each 1 per 
cent that the residue on the No. 200 sieve is reduced; because if 
coarse particles are ground fine, the whole is made up more 
nearly of particles of the same size, and smaller ones do not fill 
the voids between larger ones, so that the percentage of voids is 
increased. 

Fine grinding is obviously desirable to as great an extent as is 
compatible with economy. 

11. Time of Setting. —The setting time has no general signif¬ 
icance. For some purposes a quick-setting cement is desir¬ 
able, and for other purposes a slow-setting cement. Standard 
specifications prescribe that the initial set shall not develop in 
less than 45 minutes when determined by the Vicat needle, or 60 
minutes when determined by the Gillmore needle; 2 and that final 
set shall be attained within 10 hours. After final set, cement 
continues to harden for a long time. There is no relation between 
setting time and strength at 7 or 28 days. 

The setting time, as tested, depends upon the temperature, the 
manipulation in testing, and particularly upon the plasticity of 
the pat tested, so that it is necessary to make the test always with 
a standard amount of water used in mixing the cement, or 

1 Fine grinding often lowers the neat strength, while increasing the strength 
of mortars. 

See Meade, in A.S.T.M., p. 408, 1908. 

See also Abrams: “Effect of Fineness of Cement,” A.S.T.M., pt. II, 
p. 328, 1919. 

2 The U. S. Gov’t; specification specifies only the Gillmore needles, and 
puts the initial set at not less than 45 minutes. 

The Gillmore needle is named after Gen. Q. A. Gillmore, U. S. A., one 
of the early students of Cement, who in 1863 published a book entitled 
“Practical Treatise on Limes, Hydraulic Cements and Mortars,” probably 
the earliest extended treatise on the subject in the United States. 


CALCAREOUS CEMENT AND CONCRETE 


161 


gaging, as this determines the plasticity. A wet mixture will 
obviously set slower than a dry mixture (“mix,” as it is called). 
Seasoning sometimes increases and sometimes decreases the time 
of set. 

The test of setting time is made by mixing a batch of “neat” 
cement paste (i.e., with no admixture of sand) with a standard 
amount of cement and of water, and (when using Gillmore 
needles) forming a pat about 3 inches in diameter and inch 
in thickness, and keeping it in moist air at a temperature as near 
70° F. as practicable. The Gillmore needles are, one with a 



diameter of ^2 inch loaded with a lead weight of J4 pound, and 
the other with a diameter of 3^4 inch loaded to weigh 1 pound. 
The initial set is assumed to be reached when the pat will^bear, 
“without appreciable indentation,” the larger needle; and the final 
set when it will bear the smaller and heavier needle without appre¬ 
ciable indentation. It is not easy to tell what is meant by 
“appreciable indentation,” and this has led to the use of the 
Vicat apparatus, in which a needle 1 millimeter in diameter and 
6 centimeters long is attached to the end of a larger rod weighing 












































162 


STRUCTURAL ENGINEERING 


300 grams and mounted in a frame with a graduated scale (see 
Fig. 44). The pat is molded in a hard-rubber ring 7 centimeters 
in diameter at the base and 4 centimeters high resting on a glass 
plate, and placed under the needle, which is carefully brought in 
contact with the surface. Initial set is said to have occurred 
when the needle does not penetrate below a point 5 millimeters 
above the glass plate in minute after being released; and final 
set, when the needle “does not sink visibly into the paste.” 

Normal consistency of paste is also determined with the Vicat 
apparatus, and is defined as that consistency when the Vicat rod 
1 centimeter in diameter (not the needle) sinks to a point 10 
millimeters below the original surface in minute after being 
released. 

12. Soundness. —A cement is unsound if it expands after 
setting, thus causing cracking or disintegration. Soundness is 
obviously a most important and necessary quality. The princi¬ 
pal cause of unsoundness is free lime which does not hydrate until 
after set has occurred. 

The presence of free lime may be due to an excess in the original 
composition, above the amount necessary to form the silicates 
and aluminates which cause the set; or to failure to mix the raw 
materials before burning so as to form a homogeneous mixture; 
or to a failure to burn at a temperature sufficient to form the 
silicates and aluminates. 

If the free lime were in its ordinary condition and accessible to 
the water used in mixing, it would hydrate at once, before setting. 
Failure to so hydrate may be due to coarse grinding, the water of 
mixing gradually penetrating the coarse particles and hydrating 
the free lime there after setting has occurred, especially if the 
surface of the coarse particles is partly coated with vitrified 
clinker. Soundness is thus promoted by fine grinding, partic¬ 
ularly if accompanied by seasoning, which gives opportunity 
for all free lime to hydrate before mixing. Failure to hydrate 
quickly may also be due to too much magnesia, for magnesia, 
especially after burning, hydrates very slowly. It is mainly for 
this reason that the amount of magnesia is limited to 5 per cent. 
An excess of sulphur is also thought to cause unsoundness, by 
the formation of a compound which crystallizes slowly. Sulphur, 
however, by retarding the set, gives opportunity for more com¬ 
plete hydration before setting, so that a small amount is allowable 
and even beneficial. 


CALCAREOUS CEMENT AND CONCRETE 


163 


Specifications require that a “pat of neat cement shall remain 
firm and hard, and show no signs of distortion, checking, or disin¬ 



tegration, in the steam test for soundness.” The steam test is 
what is called an “accelerated test,” because its object is to obtain, 


Fig. 45. 






164 


STRUCTURAL ENGINEERING 


by a short test, the same results that could only be obtained by 
a very much longer test under practical conditions, since cement 
or concrete would not in practice be exposed to steam. 

The steam test of the A.S.T.M. is described as follows: 

A pat from cement paste of normal consistency about 3 inches in 
diameter, inch thick at the center, and tapering to a thin edge, 
shall be made on a clean glass plate about 4 inches square, and stored in 
moist air for 24 hours. In molding the pat, the cement paste shall 
first be flattened on the glass and the pat then formed by drawing the 
trowel from the outer edge toward the center. 

The pat shall then be placed in an atmosphere of steam at a temper¬ 
ature between 98° and 100° C. upon a suitable support 1 inch above 
boiling water for 5 hours. 

Should the pat leave the plate, distortion may be detected best with 
a straight-edge applied to the surface which was in contact with the 
plate. 

Figure 45 shows the appearance of pats that fail in the sound¬ 
ness test. 

Formerly it was also required that pats should be observed for 
soundness, one after 28 days in moist air, and one after 28 days 
in water at 70° F. The U. S. Government specifications still 
require that three pats be tested; they are all kept in moist air 
for 24 hours, then one is kept in air and a second in water at about 
70° F. for 28 days, while the third is exposed to steam for 5 hours; 
if it fails to meet the steam test, the cement may be rejected, or 
retested after 7 or more days. There has been much discussion 
and difference of opinion regarding the steam test, because some 
cements remain sound in air or water but not in steam, while in 
other cases the reverse is true. 

13. Chemical Composition.—Specifications require that the 
following limits shall not be exceeded: 


A.S.T.M. u. S. Gov’t 


Loss on ignition, per cent. 4.00 4.00 

Insoluble residue, per cent. 0.85 1.00 

Sulphuric anhydride (S0 3 ), per cent. 2.0 1.75 

Magnesia (MgO), per cent. 5.0 4.0 


A greater loss on ignition would suggest underburning or adultera¬ 
tion by calcareous material, and a greater insoluble residue might 
mean improper materials or adulteration by siliceous material. 
The significance of S0 3 and of MgO has already been alluded to. 






CALCAREOUS CEMENT AND CONCRETE 


165 


Three per cent of Ca S0 4 gives about 1.75 per cent S0 3 , so that 
these limits are usual. 

The precise chemical composition desired depends upon the 
precise silicates or aluminates to be formed; and as there are 
several of these in which the lime, silica and alumina are in 
different proportions, all of which set, though some better or more 
quickly than others, it follows that the proportion of elements 
may vary considerably without materially affecting the excellence 
of the cement. A normal American Portland cement which 
meets the standard specifications has usually a composition 
within the following limits: 

Per Cent 

Silica (Si0 2 ). 19-25 

Alumina (A1 2 0 3 ). 5- 9 

Iron oxide (Fe 2 0 3 ). 2-4 

Lime (CaO). 60-64 

Magnesia (MgO). I -4 

A good cement, however, may have a composition outside the 
above limits; and a cement within these limits may be poor. 
Defective cement more often results from imperfect manufacture 
than from faulty composition. 

Based on the desire that certain definite silicates and alumin¬ 
ates should be formed in burning, various chemists have proposed 
limits for the various constituents. Thus Newberry, an eminent 
cement chemist, assuming that the tri-silicate (3CaO, Si0 2 ) 
and the di-aluminate (2CaO, A1 2 0 3 ) were the most basic com¬ 
pounds which could exist in good cement (i.e., the compounds 
with the greatest proportion of lime) gave the formula: 

Max. per cent CaO = 2.8 times the per cent Si0 2 + 1.1 times 

the per cent A1 2 0 3 , 

or, for the cement the composition of which is given in Art. 7 
CaO = 2.8 X 21.08 + 1.1 X 7.86 = 67.67 

so that this cement would have less lime than needed to use up 
all the silica and alumina in producing the compounds named. 1 

1 “The Constitution of Hydraulic Cements’’; N. Section, Soc. of 
Chemical Industry, Oct., 1897. Though Newberry assumed that the 
dicalcic aluminate should be taken, some chemists think the tncalcic alumi- 
nate (3CaO, A1 2 0 3 ) is the most effective in setting. If this is true, New¬ 
berry’s formula would become CaO = 2.8Si0 2 + 1.65A1 2 0 3 . ^ 

The tricalcic aluminate, in hydrating, agglomerates into balls “which 
hydrate on the exterior to hard masses which prevent the penetration of 







166 


STRUCTURAL ENGINEERING 


The so-called “hydraulic index” is 


SiC >2 + AI 2 O 3 

CaO ’ 


or in the 


above example 


28.94 

63.68 


0.45. This hydraulic index has been 


used for classifying cements, the smaller the index the less the 
hydraulic activity. But this is defective, for it does not allow for 
magnesia or iron oxide, and assumes that silica and alumina are 
equivalent in giving hydraulic activity, which is not the case. 
This defect has led Eckel to use the “cementation index”; 

^ . T j 2.8Si0 2 + I.IAI 2 O 3 + 0.7Fe 2 O 3 

Cementation Index = -C^ O + 1.4MgO --- 

He says that the ideal index for a Portland cement is 1.00. 

Such indices, however, not only involve assumptions that are 
as yet unproved, but they can only be used as rough guides, be¬ 
cause the action of a cement depends not only upon its composi¬ 
tion but more on the conditions of its manufacture. It makes 
little difference whether a cement has just the proper chemical 
composition if it is underburned, or overburned, or if the materials 
have not been sufficiently mixed before burning. 

14. Mortars.—Neat cement is seldom used except for filling 
the edges of mortar joints in stone masonry (pointing). It is 
for other purposes always mixed with sand or some similar fine 
material to form a mortar. The composition of the mortar is 
expressed by the ratio of cement to sand, as 1:3. The proportion 
is in practice by volume, but in the standard tests it is always by 
weight. As the weight of a given volume depends on its com¬ 
pactness, amount of moisture, etc., these factors should be stand¬ 
ardized for testing; and in practice it is customary to specify 
a stated volume of sand for each bag of cement. 

A mortar will be strongest when it is most dense and when the 
cohesion of the cement and its adhesion to the sand grains, are 
greatest. The theory of a mortar is that the wetted cement 
shall completely fill the voids in the sand, and shall completely 
coat each sand grain. 

The voids in a mass of equal spheres, so piled to make the voids 
a minimum, are 26 per cent of the volume, and this is inde¬ 


water to the interior; consequently, large masses of unhydrated material 
are present” (Bates and Klein). 

No doubt the tricalcic aluminate, the dicalcic aluminate, the tricalcic 
silicate, and the dicalcic silicate all play a part. The hydration may set 
free an excess of hydrated lime, which, if it can be dissolved or washed out, 
may lead to disintegration. 






CALCAREOUS CEMENT AND CONCRETE 


167 


pendent of the size of the spheres. Sand grains are not spherical, 
nor of equal size, and cannot in practice be piled to produce this 
percentage of voids. The actual voids will be greater, sometimes 
up to 50, and averaging perhaps 30 to 40 per cent. If the 
cement is to coat each sand grain, the latter must not touch, but 
must be spread apart; hence the volume of cement paste must 
exceed the volume of voids in the sand if piled alone. 

The surface area to be coated will be greater the smaller the 
grains. In a mass of equal spheres, the total surface area would 
be doubled if the diameter of grains were halved; the surface 
varies inversely as the diameter. 

For these reasons, with a given proportion of cement, a coarse 
sand will give a stronger mortar than a fine sand. The greatest 
strength and economy will be obtained with a sand of mixed 
sizes. If the sand were graded so that the next to the largest 
size would just fit into the voids in the largest size, and so on 
down, the percentage of voids would be least. 

The actual percentage of voids in any sand which is to be used 
should be determined, and the ratio of cement based upon it. 
The voids should not be found by pouring water into a vessel 
containing a known volume of sand, because some air will be 
retained in the sand; but by means of the specific gravity (see 
Taylor and Thompson, page 165) or by dropping the sand into a 
given volume of water. 

15. Strength of Cements and Mortars. —The strength 
measured is either the tensile or the compressive strength. The 
standard test is of the tensile strength, notwithstanding that 
cement or mortar is never used in tension, or, if it acts in tension, 
its tensile strength is not relied on because, on account of shrink¬ 
age due to contraction in setting or to temperature changes, fine 
cracks are always possible. It has been assumed that the tensile 
strength gives at all events a measure of the excellence of the 
cement and its ability to resist compression, although there is no 
constant ratio between tensile and compressive strength. A test 
of compressive strength would be more logical. 

The form of test piece for tensile tests (“'briquette”) is shown 
in Fig. 46, designed so that the clips may grip the ends. Figure 
47 shows an automatic testing machine. The upper bucket is 
filled with shot till it balances the weight on the right, the speci¬ 
men is placed in the clips, then the shot is allowed to run out, 
when the excess weight on the right pulls the specimen, through 


168 


STRUCTURAL ENGINEERING 


the lever system. When the briquette breaks, the flow of shot is 
automatically stopped, and from the weight which has flowed 
out the tensile pull is determined. If the clips do not grip the 
specimen centrally, or if the pulls are not in the same line, there 
is an eccentric load. 



The ultimate strength will depend on many circumstances, such 
as the character of cement and sand, the consistency, the manipu¬ 
lation in mixing, the age, etc. All these things are therefore 
standardized. 1 The standard sand is one from Ottawa, Ill., 

1 See “Standard Specifications for Cement,” A.S.T.M . Standards , p. 530, 
1921. 














































































CALCAREOUS CEMENT AND CONCRETE 


169 


furnished by the Ottawa Silica Co., screened to pass a No. 20 
sieve and to be retained on a No. 30 sieve. 



Fig. 47.—Riehl6 cement testing machine. 


The A.S.T.M. specifications require the following strength: 




Tensile strength. 

Age at test, 

Storage of briquettes 

pounds per 

in days 


square inch 

7 

1 day in moist air, 6 days in water. 

200 

28 

1 day in moist air, 27 days in water. 

300 


The strength specified is the average of three briquettes of 
composition 1 cement to 3 standard sand. Formerly neat cement 
briquettes were also tested, and these are still required by the 
Gov’t specifications, 1 which prescribe: 

Pounds per 
Square Inch 


Neat cement 7 days (1 in air, 6 in water). 500 

28 days (1 in air, 27 in water). 600 

1 : 3 mortar 7 days (1 in air, 6 in water). 200 

28 days (1 in air, 27 in water). 275 


1 The U. S. Gov’t specifications, which differ in some respects from those of 
the A.S.T.M., are published as Cir. No 33, of the Bureau of Standards. 



















170 


STRUCTURAL ENGINEERING 


It is also required that the average strength of mortar 
briquettes at 28 days shall be greater than that at 7 days. 

The reason why the neat test has been given up by the 
A.S.T.M. is probably that cement is never used neat (see above) 
where strength is required, and that the neat test gives no indi¬ 
cation of the strength in mortars, which depends upon adhesion. 

The detailed treatises give full information regarding the 
strength of cements and mortars, and their variation with the 
various factors which affect it. 

16. Concrete.—Concrete is a mixture of mortar with a “ coarse 
aggregate,” consisting of broken stone, gravel, slag, or cinders. 
It is thus a mixture of cement with a fine aggregate (sand or stone 
dust) and a coarse aggregate. The fine aggregate may be con¬ 
sidered to be the particles less than H inch in diameter, or which 
will go through a sieve with H -inch openings; the coarse aggre¬ 
gate, that which is coarser than this. The coarse aggregate is 
sometimes screened to less than inches. The proportions are 
generally stated by volume, as 1:3:5, meaning 1 volume of 
cement, 3 of fine aggregate and 5 of coarse aggregate; or the 
number of cubic feet of sand and of stone per bag of cement are 
stated. The limiting sizes of stone must be specified. 

The theory of concrete is similar to that of mortar. There 
should be mortar enough to fill the voids of the large aggregate 
and to coat all the pieces of it. The greatest strength and 
economy will result, when there is the least cement to secure 
this result, since cement is the most costly ingredient; and will 
be when the aggregate is graded so that small stones occupy, as 
fully as possible, the voids between the larger stones. 

17. Proportioning Concrete.—Clearly the strength and economy 
are vitally dependent upon proper proportioning of the 
ingredients, which are: cement, sand (or fine aggregate), coarse 
aggregate, and water. The object is to make the most dense 
concrete with the least cement consistent with the principles 
above stated. 

Proportioning by Voids .—This method consists in measuring 
the voids in the dry materials, supplying cement enough to fill 
the voids in the sand (with a slight excess) and mortar enough 
(i.e.j sand enough) to fill the voids in the coarse aggregate (with 
a slight excess). This method is uncertain, because the voids in 
the concrete will not be the same as in the same dry materials, 
since the stones will wedge each other apart in a way that is 


CALCAREOUS CEMENT AND CONCRETE 


171 


uncertain, and since the particles must be separated in order that 
the cement may coat them. Nevertheless, with good judgment 
and experience, this method gives good results, and large amounts 
of the best concrete have been proportioned in this way, or even 
by arbitrarily assigning the proportions according to the average 
voids in the materials and the character of the work and strength 
desired. 

Proportioning by Maximum Density .—This may be done by 
making trial mixes before the work is begun, and finding the one 
which gives the smallest volume for a given volume of the 
unmixed materials. 

This may be facilitated by a mechanical analysis of the 
aggregate, which is made by passing it through a series of sieves 
from the coarsest at the top to the finest at the bottom, and 
finding the percentage by weight smaller than each given 
diameter as fixed by the openings between the wires in the sieves. 
A curve is drawn having for abscissas the diameter in inches, and 
for ordinates the total percentage smaller than each diameter. 
William B. Fuller determined, by tests, that the best material 
was one whose mechanical-analysis curve was a parabola; and 
from it he was able to tell in what proportions to mix different 
sizes of materials, which had been separated by screens so that the 
particles all lay between certain limiting diameters. 1 

Amount of Water .—The amount of water should be sufficient 
to give all that is needed for proper hydration of the ingredients, 
and to wet all the surfaces of the aggregate. Any deficiency 
below this, or any excess above, weakens the concrete; for if there 
is a deficiency the desired hydration will not completely take place, 
and if there is an excess the density may be decreased. If the 
concrete is plastic enough, however, the parts will settle into 
position, and will expel the surplus water, which will rise to the 
surface, so that it will not form a void of appreciable size; but 
the water which makes the cement wet must at first exist in the 
form of small quantities between the particles of the aggregate or 
cement, which are later absorbed in the hydration, and if in excess 
must leave small voids. The quantity of water necessary for 
complete hydration varies with the composition and the precise 

1 See Fuller, William B.; Chap. X in Taylor and Thompson 3d edition; 
also “The Laws of Proportioning Concrete,” Trans. A.S.C.E. 7 vol. 59, p. 
67; also Rafter: “On the Theory of Concrete,” Trans. A.S.C.E., vol. 42, 
p. 104. 


172 


STRUCTURAL ENGINEERNG 


compounds formed by the burning. An excess of water is better 
than an equal deficiency, but the excess should be as small as 
possible. Rich mixtures require less water than lean ones, 
because there is less aggregate and therefore, less surface to be 
moistened, that is, less water is required to produce the desired 
workable consistency. Professor Duff A. Abrams has studied 
this matter thoroughly, and thinks that the strongest concrete is 
not necessarily that having the greatest density, but is that in 
which the ratio of water to cement, by volume, is the least 
which will give the proper consistency. Very wet and sloppy 
concretes should be avoided, and also very dry concretes which 
require excessive tamping. In this, as in everything else, the 
golden mean is the best. The percentage of water giving 
maximum strength would give a concrete that would be too stiff 
to be workable. There must be an excess of water. Concrete 
should be kept moist after placing, until fully set; it should not 
be allowed to dry out. This is exceedingly important. 

In order to obtain the desired consistency, and uniform consis¬ 
tency throughout, when the same proportions of solid materials 
are used, the slump test or the flow test is used. In the slump test 
a frustrum of a cone 12 inches high, open top and bottom, 8 inches 
in diameter at the bottom and 4 inches at the top, is placed on a 
non-absorbent surface, and filled with the concrete in four layers, 
in a standardized manner. Three minutes thereafter, the mold 
is lifted off, and the slump is the amount that the height of the 
concrete decreases. The wetter the consistency, the greater 
the slump. In the flow test, a frustrum of a cone of concrete is 
made, the mold lifted off, and the table raised and dropped about 
3^ inch fifteen times in 10 seconds; the increase in diameter of the 
base measures the flow, or consistency. 1 

Professor Abrams has introduced a term, “ fineness modulus,” 
which is the sum of the percentages coarser than each sieve, 
divided by 100: the sieves used being the Tyler standard sieves 
100, 48, 28, 14, 8, 4 and the coarser sieves with openings % 
and lj^ inches. Thus, suppose the percentages held on the 
various sieves are as in the table: 

1 It has been suggested to use the flow method, when standardized, to 
determine time of set. It is obvious that the Gillmore needle tests only the 
crust. See Davis in A.S.T.M., p. 995, 1921. 


CALCAREOUS CEMENT AND CONCRETE 


173 


Sieve 

Size of 
opening 

Per cent 
held on 
sieve 

Per cent coarser 
than each sieve 

If 25 per 
cent broken 
smaller 

1.5 in. 

1.5 

0 

0 

0 

0.75 in. 

0.75 

25 

25 

0 

0.375 in. 

0.375 

41 

66 

66 

No. 4 

0.185 

29 

95 

95 

8 

0.093 

5 

100 

100 

14 

0.046 


100 

100 

28 

0.0232 


100 

100 

48 

0.0116 


100 

100 

100 

0.0058 

100 

100 

6.86 

= Fineness modulus 

100 

6.61 


The fineness modulus is 6.86. This modulus gives an indication 
of the fineness of the material; the larger the modulus, the coarser 
the material. This will be obvious when we observe that if in 
the above analysis the 25 per cent held on the 0.75 inch sieve 
were broken up so that it would all go through this sieve but be 
held on the one below, the column would read as in the last 
column, and the modulus would be 6.61. The coarser the 
aggregate, the smaller the total surface to be wet, and the smaller 
the amount of water required. 

Professor Abrams concludes that the sieve analysis of the 
aggregate may vary greatly without affecting the strength of the 
concrete, that the proper basis for proportioning the aggregate 
is the fineness modulus, and that “the size and grading of the 
aggregate and the quantity of cement are no longer of any 
importance except in so far as these factors influence the quantity 
of water required to produce a workable mix.” 1 This does not 
mean that the quantity of cement is of no importance, for, of 
course, the cement must fill the voids of the aggregate and 
surround each particle of it. But with any given ratio of cement to 
total aggregate it is clearly possible to grade the aggregate so as to 
produce a minimum of voids, and a maximum strength. Assum¬ 
ing that the material of the aggregate is stronger than the cement 
alone, it is easy to see that the greatest strength in compression 

1 See Abrams, D. A.: “Design of Concrete Mixtures,” being Bull. 1 of the 
Structural Materials Research Laboratory , Lewis Institute, Chicago, 1919. 















174 


STRUCTURAL ENGINEERING 


may occur with more of the stronger material (aggregate) rather 
than with the maximum density, and this is what Professor Abrams 
has found. It would not be true in tension. Professor Abrams 
assumes a given ratio of cement to total aggregate, and shows 
how to proportion the aggregate to produce maximum strength. 

Proportioning by Surface Areas. —L. N. Edwards advises the 
proportioning of mortars and concretes by surface areas of 
aggregates, 1 based on the principle that “the physical properties 
are primarily dependent upon the relation of the volume of 
cementing material to the surface area of the aggregates.” 
In other words, his tests indicated, with sufficient closeness, that 
all mixtures having the same ratio of cement to total surface 
would have the same strength. (But Professor Abrams does not 
find this to be true in his tests.) The surface areas were found 
by counting the number of particles per gram, and from the 
specific gravity determining the average size. Assuming, then, a 
sand and a coarse aggregate of given mechanical analysis, and 
assuming a given area of surface for one gram of cement, the total 
surface area is estimated and the proportions of sand and coarse 
aggregate are founds by means of curves and tables. 

The main point of the recent studies has been the importance 
of the ratio of water to cement. Assuming a given consistency, 
which should be as stiff as the nature of the work permits, and 
assuming also a given ratio of cement to total aggregate, the grad¬ 
ing of that aggregate will clearly determine the strength; and, in 
compression, the larger and coarser the aggregate (that is, the 
greater the fineness modulus) the greater in general will be the 
strength, at least up to a certain limit. This is easily seen by 
considering one particular large piece of broken stone or gravel 
aggregate; if this were broken up into a number of small pieces 
within the same exterior surfaces (discarding some pieces neces¬ 
sarily) there would be some voids within a surface where pre¬ 
viously there were none, and the total surface would also be 
increased; hence, with the same total cement in the mass, if the 
voids were just filled before, they would not be filled afterward, 
and the strengt i would be decreased. An improvement in 
grading the aggregate may double the strength. The greater the 
fineness modulus, the larger, in general, the percentage of voids 
and the less the total surface area; hence, the richer the mix (■ i.e., 

1 A.S.T.M. , pt. II, p. 235, 1918; also Young, in A.S.T.M., pt. II, p. 444, 
1919. 


CALCAREOUS CEMENT AND CONCRETE 


175 


the larger the ratio of cement to total aggregate) the larger the 
fineness modulus for the greatest strength. 

It is thus obvious that in proportioning concrete, voids, fineness 
modulus, surface area, and proportion of water, all come into 
consideration. 

The Joint Committee on Standard Specifications for Concrete 
and Reinforced Concrete, 1 in its tentative report of June 4, 1921, 
has made proportioning definite by giving tables of proportions 
to be used. Knowing the size of the coarse and fine aggregates, 
and the consistency as shown by the slump test, the table gives 
the proportions of cement, fine aggregate, and coarse aggregate 
required to produce a concrete having any desired compressive 
strength at 28 days, from 1,500 to 3,000 pounds per square inch. 
The reader should study the report of this Committee, and the 
other references given in this chapter. 2 

18. Strength of Concrete.—Concrete is assumed to act only 
in compression and shear. Its tensile strength, though not 
negligible, is ostensibly disregarded on account of the fact that 
there may be shrinkage cracks; yet its tensile strength is not and 
cannot be entirely neglected, because where there is pure shear 
there must be tension, and if shearing strength is depended upon 
(except where it is merely an oblique shear accompanying com¬ 
pression), so must also tensile strength be depended upon. 

The strength is very variable, depending upon composition, 
manipulation, character of aggregate, and age. The first Joint 
Committee, in its final report of 1916, recommended as maximum 
ultimate values to be used in design, for compression: 


Ultimate Compressive Strength of Different Mixtures of Concrete 
(Pounds per square inch at 28 days) 


Cement: aggregate. 

1:3 

1:4.5 

1:6 

1:7.5 

1:9 

Granite, trap rock. 

Gravel, hard limestone and hard 

3,300 

2,800 

2,200 

1,800 

1,400 

sandstone. 

3,000 

2,500 

2,000 

1,600 

1,300 

Soft limestone and sandstone... . 

2,200 

1,800 ! 

1,500 

1,200 

1,000 

Cinders. 

800 

700 ! 

600 

500 

400 


The allowable stresses were given as percentages of the above ultimate values. 


1 This Committee was organized in 1920, as successor of a similar com¬ 
mittee organized in 1904, which presented progress reports in 1909 and 1912, 
and a final report in 1916. It represents five technical societies. Its 
report of 1921 was published in the Proc. A.S.C.E ., Aug., 1921. 

2 See also Talbot, A. N.: “Strength and Proportioning of Concrete.” 
A.S.T.M p. 940, 192L 













176 


STRUCTURAL ENGINEERING 


Professor Abrams found the compressive strength of cylinders 
6 by 12 inches, composition 1:5, at 28 days, stored in damp sand, 
to average 2,990 pounds per square inch when of the best consist¬ 
ency, with a variation from 2,680 to 3,300. The mechanical 
analyses were very different, but the fineness ratio of the aggre¬ 
gate was constant at 6.04. The surface area per gram of cement 
varied from 5.6 to 31.3 square inches, averaging 13.8. 

With a given ratio of cement to total aggregate, the compres¬ 
sive strength will increase with the fineness modulus up to a 
certain point, and with the maximum size in the aggregate. 

The strength of a concrete may be increased by using more 
cement, or by improving the grading of the aggregate. With this 
should go a reduction in the amount of water per unit of cement. 

Probably the question of an excess of water has something to 
do with the relative amounts of crystalline and amorphous 
hydrates formed, and affects the strength in this way, though the 
relation does not seem to have been worked out. 

Tensile Strength .—The ratio of compressive to tensile strength 
is not constant, but increases with age and with the percentage of 
aggregate. As a rough average the tensile strength may be taken 
as one-tenth the compressive strength. 

Shearing StrengthA —The shearing strength is difficult to deter¬ 
mine, and tests are at variance. Pure shear, without normal 
stress, is always accompanied by pure tension of the same 
intensity on a plane at 45°, so that the pure shearing strength at a 
given point cannot exceed the pure tensile strength. Talbot 1 
found the shearing strength of 1:3:6 concrete at 60 or 61 days, 
stored in air to be 679 pounds per square inch; and at 61 to 69 
days, stored in water, to be 729 pounds per square inch. He 
says “It appears that the shearing strength is, in general, at 
least 50 per cent of the compressive strength, and that it may 
exceed 75 per cent. Evidently the shearing strength of concrete 
is several times its tensile stength.” The apparent discrepancy 
here is because in tests the average stress over a plane surface is 
obtained and not the stress at any one point. In tension over a 
plane surface, a particle of aggregate lowers the strength, because 
only the adhesion acts, and this is less than the tensile strength 
of the cement; while in shearing, a particle of aggregate greatly 
increases the strength, because the shearing strength of the stone 
is large. 

1 See Bull, 8, University of Illinois Expt. Station, 1906. 


CALCAREOUS CEMENT AND CONCRETE 


177 


Bond, or Adhesion .—In reinforced concrete it is very important 
that there should be adhesion between the concrete and the rods 
which take the tensile stress. These rods are in some cases plain 
round or square rods, and in other cases the surface is deformed, 
by ribs or projections, to make it more difficult to pull the rod 
out of the concrete, or, in other words, to give more bond. The 
bond per square inch of surface of rod varies according to the 
shape of the rod, the character of the concrete, the age, and the 
manner in which the concrete has been placed around the rod. 1 
Talbot found for plain round rods a bond of 350 to 450 pounds per 
square inch at 60 days, higher for rich concrete than for lean (as 
would be expected); the concrete being of broken stone. The 
bond will be better the more closely the concrete settles or is ram¬ 
med around the rods, that is with a wetter concrete than would 
be desirable for maximum strength of the concrete alone. 
The Joint Committee report of 1921 allows a working bond stress 
for plain bars of 0.04, and for deformed bars 0.05, of the ulti¬ 
mate compression at 28 days, or respectively one-tenth and one- 
eighth of the working compression in beams; hence, with the same 
factor of safety in both cases, it assumes the ultimate bond stress 
for plain bars one-tenth, and for deformed bars one-eighth of 
the ultimate compression. 

Tests of bond strength are made by imbedding the rod in a 
prism of concrete and pulling it out, by pushing on the prism; or 
by pushing the rod out (or starting it); or by pulling a rod by 
beam action, the rod being exposed along its length except at each 
end, where it is imbedded in the concrete. By the first method, 
the compression on the face of the block causes a lateral expansion 
according to Poisson’s ratio, which makes the concrete grip the 
rod more firmly than it would do under ordinary conditions, so 
that the bond strength found is greater than its true value. Most 
bond tests, however, are by this method. Withey 2 made tests 
by the third method, measuring the stress in the rod by exten- 
someters; and also by the first method. The ratio of the average 
bond by the first method (cylinder) to that by the beam method 
was from 1.42 for plain %-inch rods to 2.99 for 1-inch rods. 
Professor H. C. Berry 3 made tests by all three methods, but 

1 See Bull. 8, University of Illinois. 

2 A.S.T.M ., p. 467, 1908. 

3 A.S.T.M., p. 497, 1909. 


178 


STRUCTURAL ENGINEERING 


modified the beam method so as to avoid finding the stress in the 
rod by extensometers, by merely cutting the beam into two 
halves at the center of the span and. placing a hinge near the top, 
so that the stress in the rod was the moment divided by the dis¬ 
tance from the rod to the hinge. 

19. Change of Strength with Age. 1 —We have seen that con¬ 
crete sets in a few hours, and continues to harden afterward, 



Fig. 48.—Compression tests of concrete. ( Lewis Inst. Tests, 6 X 12 inch 
cylinders, Series No. 93.) 

Abrams on Strength of Concrete, Am. Soc. for Testing Materials, vol. XVIII (1918) 
part II, pp. 324, 329. 

probably for a long time. The set is due to the hydration of the 
constituents, the aluminates setting first. The hardening is due 
to progressive crystallization, to hardening of the amorphous 
material, and very likely to the gradual crystallization of 
material originally amorphous. The strength should increase 
continuously, but not at the same rate, without retrogression. 
Tensile strength increases up to 7 days, and more slowly to 28 

1 See a most interesting paper by Professor D. A. Abrams, on “The 
Effect of Age on the Strength of Concrete”; A.S.T.M ., p. 318, 1918. 































CALCAREOUS CEMENT AND CONCRETE 


179 


days or longer. It has generally been concluded, from the usual 
tensile tests, that there is a retrogression in strength from this 
point, followed by a later maximum. Professor Abrams’ studies, 
however, lead to the conclusion that this is due to the form of the 
tension test specimen and the method of loading, and that the 
real strength continues to increase indefinitely, so long as the 
concrete does not dry out. Compression tests show this continuous 
increase, without retrogression, up to the longest period covered 



Abrams on Strength of Concrete, Am. Soc. for Testing Materials, vol. XVIII (1918) 
part II, pp. 324, 329. 

(9 years), the relation between age (a) and strength (S) being of 
the form S = n log a + k where n and k are constants. It is 
very important, in order that the strength may increase, that 
the concrete should be kept wet, which suggests that the in¬ 
creased strength is due to a slow progressive hydration. If the 
concrete dries out, there is no further increase of strength. 

The explanation given by Professor Abrams for the apparent 
retrogression shown by tensile tests is that on account of the form 
of the briquette (Fig. 46) the stress is not uniformly distributed 





















































180 


STRUCTURAL ENGINEERING 


over the smallest section, but is greatest at the edges, as has long 
been recognized; and that at early ages, when the modulus of 
elasticity is low, there is more opportunity of adjustment of 
stress, and hence a smaller ratio of maximum to mean stress, than 
at later ages when the modulus of elasticity is higher. The tensile 
test gives only the mean stress, and it follows that as the age 
increases, though the mean stress may diminish, the maximum 
stress may increase. This is obviously sound. Figures 48 and 
49 show curves of increase of strength. 

Unwin gave the formula S = a + b(A — 1) OT where S is the 
strength, A the age in weeks, a the strength at seven days, b 
and m constants for any given cement. He found from Grant’s 
tests for neat Portland cement; S = 363 + 48-v^A — 1; for ce¬ 
ment mortar, 1:1, S = 157 -f 40-v^A — 1. He believed, how¬ 
ever, that a retrogression occurred after a certain maximum 
was reached. 

20. Regaging or Retempering.—It is usually specified that 
after concrete has partly set it shall not be remixed and the set 
broken. It is found, however, that if this is done, provided the 
remixing is thorough and to a uniform consistency, the concrete 
will set and will ultimately attain about the same strength as if 
the original set had not been broken, but at a slower rate. In 
other words, the crystallization and hardening can still go on. 
This is in agreement with the facts above stated, that cement 
continues to hydrate and harden even for very long periods. 
(The same is true in a way, as we have seen, of steel, where the 
amorphous dust formed by slipping hardens with time and 
becomes as hard as the original metal; that is, the elastic limit is 
raised to the previous stress and the elasticity is restored with 
time.) At the same time, it is not advisable to break up an 
initial set, particularly if early strength is desired. 

21. Rodding.—This means repeatedly pushing a pointed rod 
into concrete while it is setting. This has been studied by 
Professor F. E. Giesecke, 1 who finds that, by rodding, the 
strength may be increased as much as 100 per cent. The effect 
of rodding is to allow entrapped air and water to rise to the 
surface, and to compact the concrete; while in tamping, the upper 
portion only is compacted, and the escape of air and water below 
may be retarded or prevented. The rodding may be continued 
long after the initial set has taken place, but the crystallization 

i A.S.T.M. , p. 219, 1920; and p. 1008, 1921. 




CALCAREOUS CEMENT AND CONCRETE 


181 


and the hardening of amorphous material can still continue, as 
above shown. Professor Giesecke has rodded 1:2:4 gravel con¬ 
crete every 15 or 30 minutes for 4 hours, at which time the rod 
would not penetrate more than 2^ inches; but the specimens 
had a compressive strength of 4,211 pounds per square inch at 
28 days and 4,363 pounds at 3 months. The rodding should, 
however, not be continued so long that the cavity produced 
by the rod will not close. The relative increase of strength 
produced by rodding was greater for lean mixes than for rich, 
as could be foreseen. 

22. Modulus of Elasticity. 1 —The modulus of elasticity of 
concrete must necessarily be very variable, depending upon 
proportions, age, and other factors. The stress-strain diagram 
is probably concave towards the axis of strain. The value varies 
from about 1,500,000 to 5,000,000 pounds per square inch. For 
reinforced concrete work it is generally taken at one-fifteenth 
that of steel, or 2,000,000. 

23. Poisson’s Ratio.—This is also very variable. It is not 
often necessary to use it, but when it is, its value may be taken 
as 0.08 for a 1:3:6 mix, and as 0.18 for a 1: 1%:3J4 mix. 2 

24. Coefficients of Expansion.—The coefficient of expansion of 
concrete averages about 0.000006 per degree Fahrenheit, while 
that of steel averages 0.0000066. The close agreement of these 
makes it possible to imbed steel in concrete without the danger 
of disintegration due to differences in expansion under changes 
of temperature. 

Concrete also expands or contracts in hardening. This must 
be due to the changes which take place in the cement, as the 
aggregate does not change; consequently, the changes must be 
less the larger the proportion of aggregate, and greatest in neat 
cement. Concretes hardening under water expand, and those 
hardening in air contract, but in less degree, and if kept suffi¬ 
ciently wet should expand as those under water do. Most 
concrete is allowed to harden in dry air, and often is not kept 
as moist as it should be; and such concrete will contract, the 
amount depending on the consistency, the proportions, the 
temperature, and other conditions. 

25. Aggregates.—Sand for concrete should be clean and of 
coarse or mixed sizes. It is not necessary that it should be 

1 See Bull. 5, Lewis Institute. 

2 See Johnson’s “Materials of Construction,” p. 480. 


182 


STRUCTURAL ENGINEERING 


sharp. It may contain up to about 10 per cent loam, which, 
since its particles are very fine, will help to fill voids, and may in 
small quantities be beneficial, especially in lean concrete, but not 
in rich. Small amounts of organic matter, and particularly of 
tannic acid, may be very injurious. It may be detected by the 
color test, treating the dry sand with sodium hydroxide (NaOH) 
and observing the color of the filtrate. 1 The color must not be 
darker than the standard. Organic matter equal to or greater 
in quantity than 0.1 per cent of the sand may cause serious 
weakness or injury. 

Coarse aggregate is limited in size according to the structure. 
Generally pieces that will not go through a 2J^-inch ring are 
rejected. For reinforced concrete, which must settle closely 
around reinforcing bars, the largest size should go through a 
l-inch ring. For mass concrete, as for foundations, larger sizes 
are permitted, up to so-called 'plums or large stones that can be 
lifted by one man (one-man stones). 

Mica in sand or gravel is very bad, as it is in smooth flakes, to 
which there is little adhesion, and which increase the voids. 

Gravel aggregate gives a denser concrete than broken stone, 
because of the rounded form of the particles. Since the surfaces 
are smooth, however, the adhesion to them may not be as good 
as to broken stone. 

26. Permeability.—Concrete is often desired to be impervious 
to moisture. This requires the maximum density. Imperme¬ 
ability is sometimes increased by mixing with the aggregate some 
inert but very fine material. A small amount of loam is some¬ 
times a benefit. The fine material helps fill the voids, and 
sometimes, by improving the grading of the aggregate, it increases 
the strength of mortars and concretes. Hydrated lime is often 
mixed with cement to make it impervious, 2 up to about 15 per 
cent of the cement and sometimes more. Generally speaking, 
since this material is weaker than cement, it reduces the strength, 
but in lean mortars it may increase it by filling the voids and 
improving the grading. Such addition of slaked lime or hydrated 
lime is economical, and is often very effective in making concrete 
impervious. It should not be done where an excess of free lime 

1 See Cir. No. 1 of the Structural Material Research Laboratory, Lewis 
Institute, Chicago, 1917. 

2 See Abrams, in A.S.T.M., p. 149,1920, and p. 294, 1921; also Schertzer, 
in A.S.T.M., pt. II, p. 269, 1922; and Lazell, in A.S.T.M., p. 418, 1908. 


CALCAREOUS CEMENT AND CONCRETE 


183 


would be injurious, as in sea water, and generally not under water 
in any case. 

In Circ. No. 30 of the U. S. Bureau of Standards, entitled 
“Lime: Its Properties and Uses/’ the following statement is 
made: 

In a series of experiments in which the Portland cement in a mortar 
was replaced by varying amounts of hydrated lime it was found: (1) 
That hydrated lime up to 15 per cent (by weight) of the cement does not 
materially affect the strength of the mortar, even when stored under 
water; (2) this amount of hydrated lime will materially increase the 
imperviousness to water of even a 1:5 cement-sand mortar; (3) the addi¬ 
tion of hydrated lime increases the plasticity of the mortar and makes it 
easier to work. 

27. Concrete in Sea Water.—In sea water, concrete should be 
proportioned and deposited with special care, as there have been 
numerous failures. These have been due, (1) to mechanical 
action, blows, etc., as by floating ice, which have damaged the 
surface or broken off corners; (2) to perviousness of the concrete, 
which allows water to enter and afterward freeze as the tide falls, 
causing disintegration, mainly between the levels of high and 
low tide; (3) failures in sea water have been often attributed to 
chemical action of the salts in the water, especially sulphates and 
magnesia, which have been supposed to unite with some of the 
constituents of the cement, especially the aluminates, forming 
sulpho-aluminates of lime and magnesia, which crystallize with 
much water, expanding, and disintegrating the concrete. Later 
investigations and experience indicate, however, that the most 
important disintegrating influences are mechanical action and 
porosity. 1 

Low alumina cements have often been considered best for use 
in salt water, and the alumina has been limited to not over 6 
per cent. But, on the other hand, some high alumina cements 
have been used with success, and have even been considered 
superior, so that this matter must be regarded as doubtful, 
though Le Chatelier considered cements high in alumina to be bad 
for sea work. No doubt the most important precaution is to 
secure a dense and impervious concrete and to use a rich mix, 
especially for the surface layers, which should be as dense and 
inpervious as possible. Often a special mixture is placed so 

1 See Technologic Payer 12 of the Bureau of Standards for an elaborate 
study of this question; also Atwood and Johnson in Trans. A.S.C.E. for 
Aug., 1923, and discussion following. 


184 


STRUCTURAL ENGINEERING 


as to form a skin, say a foot thick, of such dense mixture. 
But hydrated lime should not in this case be used to promote 
imperviousness. 

28. Uncertainties of Concrete Construction.—Concrete con¬ 
struction is fundamentally different from construction of steel or 
any other usual material, in that the concrete is manufactured on 
the spot out of the component parts, often by unskilled labor, 
while steel is a finished product brought to the work for erection, 
after being fabricated by skilled labor. There are thus greater 
uncertainties in concrete work, in the choice of materials, propor¬ 
tioning, and preparation. Further, in the testing of cement there 
are more sources of error, due to the large number of variable 
elements. Nevertheless, concrete has taken its place as one of 
the most important engineering materials, and is used in enormous 
quantities. 

29. Microscopic Study of Concrete.—Within a few years, the 
microstructure of concrete has been studied. See a good paper 
on this subject by Nathan C. Johnson, in Proc. A.S.T.M., p. 172, 
1915, which should be read by the student. Further study of 
microstructure will doubtless help to solve some of the problems 
of setting, and will shed light on proportioning and other still 
uncertain questions, just as it has in the case of steel. 


CHAPTER XIII 


CORROSION OF METAL—PAINTS AND VARNISHES 

1. The importance of protecting wood and metal in structures 
from decay and corrosion is obvious. Protective coatings are 
engineering materials of the highest value, and worthy of just as 
careful study as those which form the structure itself. It is of 
little use to design an adequate structure if its permanence is not 
ensured by every reasonable means. 

2. Protection of Metal.—Metal is destroyed by corrosion or 
rusting, which is the formation of ferric hydroxide, Fe0 3 H 3 . 
Regarding the nature of corrosion, Prof. 0. P. Watts, in Chap. 
XXIX of Johnson’s “Materials of Construction,” says “The two 
modern theories of the rusting of iron are the Acid and the 
Electrolytic Theories. According to the former the presence of an 
acid is necessary to the formation of rust, but even so weak an 
acid as carbonic may serve. The acid causes the metal to dis¬ 
solve, and the oxygen changes the dissolved metal to rust, thereby 
liberating the acid, which is then capable of dissolving more 
metal, and so the process goes on. After several years of careful 
experimenting by different investigators, it now seems to be 
established that moisture and oxygen are sufficient for the con¬ 
tinued rusting of iron, so that the electrolytic theory of rusting is 
the one more generally accepted.” The details of the subject 
therefore belong in the field of the chemical engineer, and there is, 
as yet, much conflict of opinion regarding many matters. How¬ 
ever, Friend’s experiments seem to prove conclusively that in 
pure water free from C0 2 , but containing air, iron does not rust. 

Certain points, however, seem clear. It is agreed that for 
corrosion both moisture and oxygen must be present. Iron will 
not rust in dry air, nor in water without oxygen. It will rust 
where there is no C0 2 , but the presence of this, or of acid fumes, 
salt water, or acid waters, accelerates corrosion. Segregation or 
other lack of homogeneity in the metal also promote corrosion, 
and certain impurities are supposed to have the same effect, such 
as sulphur and manganese, the latter within certain limits. The 

185 


186 


STRUCTURAL ENGINEERING 


presence of rust itself greatly stimulates further rusting beneath. 
Nickel steel appears to be more resistant than ordinary carbon 
steel. 

It is commonly supposed that steel rusts more than wrought 
iron, and wrought iron more than cast iron; but it is probable that 
some observed differences that seem to justify this view are due 
to other conditions 1 (see Art. 12). 

Iron rusts faster in sea water than in pure fresh water, and still 
faster in sea water polluted by sewage. Cast iron, after long 
immersion in sea water, has been found to be converted into a 
soft, black substance, like plumbago, that could be cut with a knife. 

Corrosion seems to be affected by stress, strain, and vibration 
in a manner that is still uncertain. It is curious that steel rails 
in service do not rust, while the same rails, laid aside, rust rapidly. 

A coating of Portland cement seems to prevent rust. This is 
fortunate, in view of the extended use of reinforced concrete. 
It shows the importance, however, of making the concrete in 
such structures so that a film of cement will surround the steel, by 
using a rather wet mix, tamping it well, and choosing a bar of 
suitable shape. Concrete, when new, is alkaline, which prevents 
rusting. Many believe that in time it becomes neutral, and then 
protects the steel only mechanically. The protection may be 
perfect, but is not always so. Many believe that it is best not 
to paint steel bars to be used as reinforcing, and that it does not 
seem necessary to remove rust that may have formed before 
placing. This must be said to be still a doubtful question. 

Nevertheless, the question of preventing corrosion of the steel 
in reinforced concrete is an important one, especially for concrete 
to be immersed in water, particularly in sea water. Some of the 
reinforcing bars in concrete viaducts over sea water, which the 
writer has examined, have been found to be considerably rusted 
and the concrete protection broken off. Whether the breaking 
of the concrete exposed the steel and led to its rusting, or whether 
rusting of the steel caused the concrete to break off, could not be 
ascertained. 

R. A. Cummings states as his practice: “Wherever reinforcement is 
imbedded in concrete that is submerged in water or subjected to mois¬ 
ture, the rods are completely coated by immersion in a bath of neat 
Portland cement immediately before being placed in the work.” 2 

1 See Proc. A.S.T.M., p. 247, 1908 and p. 155, 1905. 

2 Proc. Eng. Soc. Western Pennsylvania, Jan., 1909. 


CORROSION OF METAL—PAINTS AND VARNISHES 187 


Galvanizing or painting with oil or tar should of course be avoided as 
reducing or destroying the adhesion of the concrete. 

3. For steelwork exposed to the air, corrosion may be prevented 
by proper paint. Therefore steel structures should be designed so 
that every part, if possible, should be accessible for inspection 
and painting. The structural engineer should know what kind 
of paint to use and how to use it. The subject is a specialized 
one, largely a branch of chemistry, but some fundamental prin¬ 
ciples should be stated. 

PAINT 

4. Necessary Qualities of Paint.—The qualities which it is 
desirable that a paint should possess are: 

It should be impervious to moisture. 

It should be as impervious to air as practicable. 

It should be somewhat elastic, so that it will not crack when 
the metal deforms under stress. 

It should be inert, containing no ingredients that will attack 
metal. 

It should be durable. 

It should not be acted on or attacked, after it has dried, by 
air or any gases likely to be mixed with air. 

It should be tough and hard, so that it will resist reasonable 
abrasion to which it may be exposed. 

5. A paint consists of a base and a vehicle, to which are some¬ 
times added solvents or thinners to make it more liquid, driers to 
hasten the hardening, and pigments to give the color desired, if 
not given by the base. The base gives opacity, color, solidity, 
imperviousness, and hardness; the vehicle gives elasticity, 
strength, and cohesiveness. 

6. The vehicle is almost always linseed oil, which, when exposed 
to the air, hardens, not by evaporation as wet bodies do, but by 
absorbing oxygen and becoming converted into a tough, elastic 
material called linoxyn. The drying is hastened by dry weather 
and sunlight, and by the use of so-called “driers,” but raw linseed 
oil generally requires 24 hours and often much longer. Raw 
linseed oil is either hot-pressed or cold-pressed: cold-pressed oil 
has a golden-yellow color, while hot-pressed oil is darker and less 
fluid, and contains some fats and solid organic matters which are 
harmful. 

Linseed oil is either raw or boiled. Boiled oil is not really 
boiled, but merely heated with certain substances, called “ driers,” 


188 


STRUCTURAL ENGINEERING 


which are oxidizing agents, such as oxides of lead or manganese, 
which transfer oxygen to the oil and so make it thicker, and 
make it dry more quickly, than raw oil. The raw oil, however, 
is more durable than boiled oil, and only small quantities of drier 
should be used. Boiled oil, however, hardens quicker and makes 
a harder film; and since water tends to soften any paint film, and 
the harder film resists this best, boiled oil is preferable for paint 
exposed to water, as for the interior of water tanks and stand¬ 
pipes; and many prefer it to raw oil for all exterior work, not¬ 
withstanding its smaller durability. The film given by boiled 
oil, however, is sometimes too hard, and more liable to crack 
than the film of raw oil. The greater thickness of boiled oil is 
often counteracted, and the paint made more workable, by 
diluting it with thinners, chiefly turpentine, which completely 
evaporates. Raw linseed oil hardens so slowly that often a drier 
must be added to the raw oil, without heating, while boiled oil 
often requires a thinner. The danger in a drier is, that since it 
is an oxidizing agent, it may, if in excess, oxidize the metal, and 
so contribute to the effect that the paint is supposed to prevent. 
The danger in a thinner is that the paint may be made so thin 
that it spreads into too thin a film, thinning the oil which gives 
it strength, and so does not give sufficient protection or durability. 
It is for these reasons that some bridge specifications ( e.g ., 
Department of Railways and Canals, Canada) prohibit the use 
of all driers and thinners. Some substances formerly used as 
driers contained rosin, which was therefore found in some com¬ 
mercial boiled oils; but within recent years rosin is not found in 
commercial boiled oil, and since rosin is injurious because it causes 
brittleness and lessens durability, it should not be allowed in 
driers or oils. 

Altogether, it seems desirable to use raw oil, with a small, 
specified, amount of drier which shall contain no rosin. If a 
harder film is desired, a mixture of boiled and raw oils may be 
used, with specified small amounts of drier and thinner; and 
for continued exposure to water boiled oil should be used. Cir¬ 
cular No 69 of the U. S. Bureau of Standards states (page 13) “This 
Bureau knows of no case in which boiled oil is used in paint, 
where equally satisfactory results could not be obtained by the 
use of raw oil and a suitable Japan drier.” Owing to the uncer¬ 
tainty of the weather, it is generally necessary to use some drier 
for out-door painting. Also, while most engineers formerly 


CORROSION OF METAL—PAINTS AND VARNISHES 189 


believed the use of a volatile thinner undesirable, probably 
because of the danger of excessive use to make the work easier 
for the painter, it is now generally believed that a proper amount 
of such thinner is desirable, particularly if boiled oil is used, and 
especially for undercoats on wood, which should be dried through 
before the next coat is put on. The proper amounts of oil, drier, 
and thinner depend on “the nature of the pigment, character of 
the surface to be painted, whether undercoat or finishing coat, 
and exposure (whether inside or outside).” In painting wood, 
the pores, if not previously filled by a filler, will absorb consider¬ 
able of the vehicle, which should therefore be in greater amount 
than for painting metal. The use of a thinner will help this 
absorption. It is often desirable to use turpentine in repainting, 
to soften the old paint and make the new paint adhere better. 
It may also be used when it is desired to have much pigment, as 
it evaporates, and so really reduces the proportion of oil, while 
keeping the paint workable. 

7. The base is an inert, opaque, durable material, which is 
spread over the surface by the vehicle. It should be finely 
ground, so that one particle may not occupy the whole thickness 
of the film. The more finely ground, the better the paint will 
spread. Red lead, white lead, iron oxides, carbon, and other 
substances, are used. The most widely used is red lead (Pb 3 0 4 ), 
which is made by heating litharge (PbO). The more finely 
ground the litharge, the greater the proportion of red lead in 
the product, and the finer that product, hence the better the 
paint. Litharge is easily decomposed, and is acted upon by 
linseed oil, while red lead is more stable and permanent. The 
base should not be acted on by the oil. Litharge in oil becomes 
ropy, and on standing hardens to a solid. If commercial red 
lead contains much litharge, it means that the litharge was not 
finely ground, the paint will harden in the can if kept, the paint 
is heavy, forms lumps, runs, and is hard to apply. The finer 
the pigment, the more it attracts the oil to bind it into a firm 
film, and the more it repels water. Formerly commercial red 
lead contained 10 to 15 per cent of litharge, and while it made a 
good paint, it had the objections that it dries quicker than pure 
red lead, but is not so durable. Lately commercial red lead can 
be obtained which contains 98 per cent or over of pure red lead. 
Paint made of this high-grade product works easily, and is very 
permanent. It is put up as a paste in cans, and does not harden. 


190 


STRUCTURAL ENGINEERING 


The U. S. Gov’t specifications require red lead for paint to contain 
at least 94 per cent pure red lead, with a higher percentage in 
some cases. 

Other bases, such as iron oxides, white lead, carbon, graphite, 
silica, cement, and others, have been used for paints on metals, 
but the general opinion of engineers is that nothing is so good as 
red lead. Generally, from 25 to 30 pounds of pure red lead is 
used to a gallon of oil, though often more red lead is used, as in 
the Hell Gate bridge, where 37.5 pounds were used to a gallon of 
oil. 1 The Metropolitan Water Board of Massachusetts has 
largely used for steel exposed to water a paint of 98 per cent red 
lead to which is added a small amount of finely powdered lith¬ 
arge, for the purpose of getting a hard coat, using also boiled oil. 

8. Varnish.—Paint is a mixture; varnish is a solution of some 
resin or gum in a liquid which evaporates and leaves the resin 
as a hard and sometimes transparent coating. The liquid is 
spirit or oil. In an oil varnish the oil forms part of the film. If 
a pigment is added to a varnish, it is an enamel paint, the varnish 
being the vehicle. Linseed oil is often used in oil varnish, and 
Chinese Tung oil in certain cases, which dries more rapidly than 
linseed, and is tough and durable. Rosin is bad in varnish, as 
in paint, making the film brittle, causing it to lack durability. 

9. Painting on Wood.—Wood can absorb some of the paint 
vehicle. It is therefore necessary first to fill the pores with a 
filler, which may be paste or liquid, the latter being more conve¬ 
nient; or else to proportion the undercoat so that some of the 
vehicle may be absorbed without injury to the film. Also, 
paint on wood may not only keep moisture out, but may keep 
moisture in and so cause decay (dry-rot). Hence paint should 
be applied only to dry seasoned wood. The best filler is linseed 
oil, sometimes thinned with turpentine if applied to resinous wood. 
Knots should be coated with shellac before painting. Common 
bases for white paint on wood are white lead and zinc white; 
many think zinc white the best, or a mixture of the two. 

10. Painting.—A good paint depends for its durability quite 
as much upon a proper preparation of the surface, and proper 
methods of application as upon proper material. 

Painting should never be done in wet or damp weather or when 
the temperature is under 50° F. (some say 40° F.). 

1 Sabin, Red Lead p. 28. 


CORROSION OF METAL—PAINTS AND VARNISHES 191 


In painting wood, the surface should be smoothed by sand¬ 
papering, and should be clean and dry. 

In painting metal, all rust, dirt, and loose mill scale should be 
removed, and all grease removed by washing with benzine or 
caustic soda. The surface should be thoroughly dry and clean. 
There are three methods of cleaning a metal surface: 

(1) By the sand blast. This is a thoroughly efficient process, 
but expensive. It leaves the surface so clean that it is particu¬ 
larly liable to rust, and it should be painted immediately after 
cleaning, to secure good results. Some engineers do not like it 
for this reason, but no matter what method is used, if thoroughly 
cleaned, the surface should be painted at once. 

(2) By pickling, which is immersion in dilute acid, after clean¬ 
ing of dirt and grease. After pickling, the acid must be removed 
by washing with an alkali. Pickling is thought by some to be 
better than sand-blasting, as it leaves the surface rough, and the 
paint adheres better, while sand-blasting smoothes the surface. 
Pickling sometimes causes a surface brittleness which is most 
marked in thin pieces. 

Pickling is of course possible only before original construction. 

(3) By far the most common method is to clean the scale and 
rust off by scraping and hammering, and the use of wire brushes. 
Very adherent mill scale may not be removed in this way, and 
many think that it does no harm to paint over such scale, while 
others deny this and claim that, if there is any rust beneath the 
paint, rusting will continue. It is doubtless better to remove all 
mill scale, for it may loosen later. Of course all loose scale 
should be removed. 

The Committee of the A.R.E.A. on Preparation of iron and steel 
surfaces for Painting reported in 1921 that scraping, wire brushing, and 
wiping gave as good results as sand-blasting or pickling where corrosion 
had not commenced or had only moderately progressed. They also 
stated that treating old painted surfaces which have bare rusted spots 
by brushing the coating with benzine, burning the benzine off, then 
scraping and wire brushing, gave better results than without the benzine 
treatment (Proc. A.R.E.A., p. 345, 1921). 

11. Number of Coats.—Steelwork should be given one coat of 
pure red lead before leaving the shop, though rarely a coat of linseed 
oil is used instead. The oil or paint film contracts in drying, 
and hence tends to draw away the film from protuberances like 
rivet heads and from the edges of angles and other shapes. 


192 


STRUCTURAL ENGINEERING 


Hence Professor Sabin recommends that before putting on the first 
field coat, all rivet and bolt heads should be painted, all places 
not well covered by the original shop coat should be repainted, 
and that all edges shall receive a narrow striping coat extending 
an inch from the edge. When this is dry a full coat should be 
given to the entire surface, and, when this is dry, a third coat. 
No coat should be applied till the previous coat is dry. 

It is also considered that wood should originally receive three 
coats. For repainting, the old paint need not be removed unless 
loose, and one or two coats may be enough. 

The covering capacity of a paint depends upon the character 
of the surface, the fineness of the base, the consistency of the 
paint, and the degree of brushing. 

The price of steelwork is estimated by the ton, and it is often 
convenient to estimate what the paint will cost per ton of steel. 
It is clear that this is an improper basis, for it is not tonnage, but 
surface, that is painted. A gallon of red lead paint of proper 
consistency will cover about 700 square feet. A ton of thin mater¬ 
ial will require more paint than a ton of thick material. Professor 
Sabin gives tables, and comes to the conclusion that “on medium 
weight bridges three-eighths of a gallon of paint per ton for the 
first coat, one-fourth gallon for the second coat, one-fourth for 
the finishing coat, or seven-eighths gallon for the three coats, 
would be sufficient ; heavier bridges less, and lighter bridges 
more; roofs 1,000 to 1,200 square feet per gallon.” 

12. Steel vs. Wrought-iron.—Reference has been made in 
Art. 1 to the common belief that steel rusts faster than wrought- 
iron. This subject is discussed in the references given. The 
writer definitely shares in this belief, though not based on scien¬ 
tific demonstration, but on his experience. The wrought-iron 
links of the suspension bridge at Newburyport, Mass., built in 
1810, which was rebuilt in 1910 by R. R. Evans, County Engin¬ 
eer, with the writer as Consulting Engineer, were in very good 
condition after 100 years of service. On the New York elevated 
railroads a steel structure for third track was built of steel in 
1916, and this had in 1923 deteriorated much more than the 
wrought-iron structures adjoining, though the latter had not 
been painted since several years before 1916. This is of course 
not conclusive, for the paint on the wrought-iron may have been 
better than on the steel, though there is no reason to suppose it 
was. Nevertheless, steel is and will be almost exclusively used 


CORROSION OF METAL—PAINTS AND VARNISHES 193 


for structures, and the question is how best to protect it; and 
many metallurgists believe that there is little if any difference 
between good wrought-iron and good steel as regards corrosion. 
There is good evidence, however, that cast-iron resists corrosion 
better than wrought-iron or steel. 

13. Copper in Steel.—There is good evidence that a small 
percentage of copper in steel greatly reduces the corrosion of 
uncoated sheets exposed to the weather. The Committee of 
the A.R.E.A. on corrosion has made many tests at Pittsburgh 
and in 1921 reported: “We may definitely conclude that copper¬ 
bearing metal shows marked superiority in rust-resisting proper¬ 
ties in comparison to non-copper-bearing metal of substantially 
the same general composition, from which superiority we may 
truly anticipate a marked increase in the service life for copper¬ 
bearing metals under atmospheric exposure of uncoated sheets” 
(Italics ours). On the other hand, when immersed in mine water 
the copper had little effect, and, if there was any, its presence gave 
a slightly shorter life. The Committee, however, thought it 
“safer not to draw definite conclusions’’ till the tests should be 
complete. However, exposed to the atmosphere, of 132 copper¬ 
bearing sheets, No. 16 gage, none had failed, while of 126 non- 
copper-bearing sheets 54 had failed, after 52 months’ exposure; 
and of 146 copper-bearing sheets, No. 22 gage, 93 had failed, and 
of 84-non-copper-bearing sheets 82 had failed after the same time. 

Professor W. H. Walker (Trans. Am. Electro-Chemical Soc. 
1921) said: 

Impartial evidence is now legion that no commercial iron or steel so 
well withstands atmospheric corrosion as does steel containing approxi¬ 
mately 0.2 per cent copper. 

Professor Samuel L. Hoyt of the University of Minnesota, in 
an article in “ Chemical and Metallurgical Engineering ,” Aug. 1, 
1919, says: 

The general conclusion which it is believed may be drawn from this 
exposure test is that so-called copper-bearing steel, in which the copper 
content is about 0.20 to 0.25 per cent, offers the greatest resistance to 
corrosion of the common sheeting materials. After copper-bearing 
steel come pure open-hearth iron, open-hearth steel, and bessemer steel 
in order of excellence. The two best sheeting materials, according to 
this test, are copper-bearing steel and pure iron. Both as regards resis¬ 
tance to atmospheric corrosion and cost of production, copper-bearing 


194 


STRUCTURAL ENGINEERING 


steel seems to possess a decided advantage over pure open-hearth iron 
as a sheeting material. 

Copper steel with about 0.25 per cent copper (0.2 to 0.3) has 
been largely used in the construction of steel cars, where corrosion 
is apt to be rapid, not only on account of the weather, but also 
on account of the sulphur contained in coal and ore. 1 

It has not been used for bridges, so far as the writer knows, 
except in the Fortieth St. bridge across the Allegheny river in 
Pittsburgh, built in 1924, in which copper-steel with about 
0.2 per cent copper was used, with an allowed variation of 0.02 
above or below. 

The additional cost of copper steel of this composition above 
ordinary structural steel is stated to be about S3 per ton (Dec., 
1923). 

14. Metallic Coatings.—Steel and iron are often protected by 
metallic coatings, either (a) of a different metal, or ( b) in which 
the surface metal itself is converted into some less corrodible 
compound. 

Of the coatings of the first class, zinc coatings are considered 
the best for protection against corrosion. Galvanized iron is 
iron or steel coated with zinc. It is stated that “the life of zinc 
coatings, particularly those of a porous character, may be 
prolonged almost indefinitely by periodically oiling them.” 2 

Of the coatings of the second class, a well-known one is that 
produced by the Bower-Barff process, or modifications of it, by 
which the metal is oxidized to Fe 20 3 by being heated in super¬ 
heated steam, or in a mixture of steam and benzine. A layer of 
oxide formed at the temper color of a- blue heat, or other color, 
which forms a layer of oxide, when heated in free air, is often 
used. 2 

Metallic coatings have little application in structural work, 
except that galvanized iron is often used for the roof covering 
and sides of buildings. 

15. References.—The above will, it is hoped, enable the reader 
to understand current specifications and the reasons for them. 
The following authorities should be consulted for further study: 

1 See Railway Age , June 16, 1923; “ Reducing the Corrosion in Steel 
Cars,” by J. J. Tatum, Supt. Car Dept., B. & O. R. R. 

2 Circular No. 80, U. S. Bureau of Standards, on “ Protective Metallic 
Coatings for the Rustproofing of Iron and Steel.” This contains a good 
bibliography. 


CORROSION OF METAL—PAINTS AND VARNISHES 195 

1. Snow: “Paints for Wood,” Chap. XV, with many references. 

2. Circular No. 69, U. S. Bureau of Standards, on “Paint and Varnish.” 

3. Sabin, A. H.: “Technology of Paint and Varnish,” John Wiley & 
Sons, Inc., 1917. 

4. Sabin, A. H.: “Red Lead and How to Use It,” John Wiley & Sons, 
Inc., 1920. 

5. Stoughton: Chap. XVI on Corrosion of Iron and Steel. 

6. Johnson: Chap. XXIX on Corrosion of Iron and Steel. 

7. Friend: “The Corrosion of Iron and Steel,” Longmans, Green & 
Co., 1911. 

8. Friend: “Chemistry of Paints,” Longmans, Green & Co., 1910. 

9. Lang: “The Corrosion of Iron and Steel,” McGraw-Hill Book Co., 
Inc., 1910. 

10. Sang: Proc. Eng. Soc. West. Pa ., Jan., 1909, with discussion. 

11. Am. Soc. for Testing Materials. Various volumes, see Index. 

WATERPROOFING 

16. The waterproofing of masonry or concrete is of impor¬ 
tance. The materials used are of two kinds; (1) fabrics, or (2) 
so-called “integral” waterproofing. Waterproofing by fabrics 
consists in using several layers of tarred felt, paper, or cloth 
saturated with a waterproofing substance. These are placed on 
the outside of the masonry or concrete, each layer swabbed with 
coal tar, asphalt, or some such substance, sometimes four or five 
layers being used, to prevent the water from penetrating the 
concrete. Integral waterproofing consists in making the concrete 
as impervious as possible, by adding to it some substance that 
will fill the pores and keep out water. Sometimes only the outer 
skin, or the inner skin, is made of this waterproof material. 
Sometimes, if a cellar wall leaks, the endeavor must be made to 
make it tight by supplying an inner layer of some waterproof 
material. 

There are a number of compounds, methods, and fabrics, 
offered for use, and the reader is referred to Sweet’s catalogue, to 
the advertisements in the engineering periodicals, and to the 
catalogues of advertisers, The Proceedings of the A.S.T.M., the 
A.C.I., and the A.R.E.A. for many papers on this subject. 

As to a comparison of the fabric with integral waterproofing, 
it is obviously better to keep the water from getting at the con¬ 
crete than it is to try to make the latter impervious, and the 
writer, in his many years of experience in constructing subways 
in Boston, has always used fabric. The Committee of the 
A.S.C.E. on Concrete and Reinforced Concrete, reporting in 
1917, said: 


196 


STRUCTURAL ENGINEERING 


When mortar or concrete is proportioned to obtain the greatest prac¬ 
ticable density and is mixed to a proper consistency, the resulting mortar 
or concrete is impervious under moderate pressure. 

On the other hand, concrete of dry consistency is more or less pervious 
to water, and, though compounds of various kinds have been mixed with 
the concrete or applied as a wash to the surface, in an effort to offset 
this defect, these expedients have generally been disappointing, for the 
reason that many of these compounds have at best but temporary value, 
and in time lose their power of imparting impermeability to the concrete. 

The Joint Committee on Standard Specifications for Concrete 
and Reinforced Concrete, representing five engineering societies, 
in its Tentative Specifications submitted June 4,1921 (see Trans. 
A.S.C.E.) says. “Integral Compounds shall not be used.” 
Nevertheless, they are useful in certain cases. 


INDEX 


A 

Accelerated test, 163 
Adhesion of concrete, 177 
Aggregates, concrete, 181 
Air hardening, 117 
Alloy steels, 108 
chrome, 115 
chrome nickel, 116 
chrome vanadium, 116 
copper, 115 

effect of heat treatment, 109 
high speed tool steel, 116 
manganese, 109 
Mayari, 116 
nickel, 112 
vanadium, 116 
Aluminum, 116, 128 
Annealing, 56 
Austenite, 40 

B 

Babbit metal, 133 
Bearing metal, 132 
Bell metal, 130 
Bending, cold, 99 
Blocks, hollow, 151 
Blow holes in cast iron, 75 
in steel castings, 105 
Bond, brick, 146 
concrete, 177 
Borers, marine, 34 
Brard’s test, 142 
Brasses, 128 
manganese, 129 
Muntz metal, 129 
Brick, 143 
bond, 146 
elasticity of, 145 
header, 146 
piers, 146 


Brick, strength of, 144 
stretcher, 146 
Briquette, cement, 167 
Bronzes, 129 
bell metal, 129 
gun metal, 129 
machinery, 130 
phosphor, 130 
Tobin, 132 
Burnettizing, 37 

C 

Case hardening, iron, 87 
wood, 8 
Cast iron, 69 
allowable stresses, 78 
castings, 74 

composition and constitution, 72 
defects, blow holes, cracks, segre¬ 
gation, 75 
definition of, 69 
gray, 70 
malleable, 85 
mottled, 71 
protection of, 79 
shrinkage of, 73 
strength of, 76 
uses of, 72 
water pipes, 78 
white, 70 

Castings, cast iron, 74 
steel, 104 
Cement, 153 
German, 157 
Natural, 157 
Portland, 157 
analysis of, 158 
cementation index, 166 
chemical composition, 164 
definition of, 157 
fineness, 159 
hydraulic index, 164 


197 


/ 


198 


INDEX 


Cement, Portland, neat, 161 
soundness, 162 
specific gravity, 158 
strength of, 167 
time of setting, 160 
Puzzolana, 157 
slag, 157 
Cementite, 40 
Checks in wood, 8, 9, 24 
Chelura, 34 
Chrome steels, 115 
Classification of materials, 3 
Cold bending, 99 
crystallization, 63 
rolling, 60 
Concrete, 170 
adhesion or bond, 177 
age, 178 
aggregates, 181 
fineness modulus, 172 
flow test, 172 
modulus of elasticity, 181 
permeability, 182 
proportioning, 170 
maximum density, 171 
surface areas, 174 
voids, 170 

regaging or retempering, 180 

rodding, 180 

sea water, 183 

slump test, 172 

strength of, 167 

water, 171 

Constitution of iron and steel, 39 
Copper, 126 
Corrosion, 185 
copper in steel, 193 
cracking, 133 
steel in concrete, 186 
steel vs. wrought iron, 192 
theories of, 185 
Creosote, 37 

D 

Decay of timber, 31 
Defects, 9, 75 
effects of, 24 
Density, 11 
Rule, 14 


Drawing of wire, 56 
Driers, 187 
Duralumin, 130 

E 

Elasticity, brick, 145 
concrete, 181 
iron, 82-83 
steel, 97 
stone, 136 
wood, 23 
Eutectic, 42 
Eutectoid, 44 

F 

Factor of safety, stone, 140 
timber, 28 
Ferrite, 49 

Fineness modulus, 172 
Flow test, 172 
Flux, 69 
Fracture, 67 

G 

r. 

Galvanizing, 194 
Gangue, 69 
German cement, 157 
Gillmore needles, 160 
Grading rules, 10 
Grain of wood, 10 
Gray cast iron, 70 
Gun metal, 129 

H 

Hammering, 61 

Hardening of iron and steel, 51 
Hardenite, 56 
Hardness, strain, 59 
Header, 146 
Heartwood, 5 

Heat treatment, effect of, 109 
meaning of, 57 
High speed tool steel, 116 
Hydrated lime, 154 
Hydraulic index, 166 


INDEX 


1 

Index, cementation, 166 
hydraulic, 164 
Ingotism, 104 
Initial stresses, 59 
Invar, 114 
Iron, 80 

cast (see Cast iron) 
corrosion of, 185 
forms of, 43 
pig, 69 

steel (see Steel) 
wrought (.see Wrought iron) 

K 

Knots, 9, 24 
Kyanizing, 37 

L 

Lead, 128 
Lime, 153 
free, 156, 162 
hydrated, 154 
hydraulic, 157 
mortar, 155 
quicklime, 153 
Limnoria, 34 
Linseed oil, 187 
Lumber, definition of, 4 

M 

Machinery bronze, 130 
Malleable cast iron, 85 
shrinkage of, 87 
strength of, 86 
uses of, 87 

Manganese, bronze, 129 
steel, 109 
Marine borers, 34 
Martensite, 51 
Mayari steel, 116 
Mechanical work, effect of, 58 
Merit numbers, 111 
Metallic coatings, 194 
Mortar, 149, 166 
Mottled cast iron, 71 
Muntz metal, 129 


N 

Natural cement, 157 
Neat cement, 161 
Newmann bands, 65 
Nickel, 128 
steel, 112-116 
Normalizing, 57 

P 

Paint, 187 
base, 189 
driers, 187 
qualities, 187 
thinners, 188 
varnish, 190 
vehicle, 187 
Pearlite, 44 
Phosphor bronze, 130 
Pig iron, 69 
grades of, 76 
Pitch products, 9 

preservation of timber, 37 
proportioning concrete, 170 

Q 

Quicklime, 154 

R 

Rodding of concrete, 180 
Rot, 9, 34 

S 

Sapwood, 5 

Seasoning of wood, 8, 37 
Segregation, cast iron, 75 
steel castings, 105 
Self hardening steels, 109 
Shakes, 8, 9, 24 
Shrinkage, 8, 105 
Slip bands, 64 
Sorbite, 51 
Springwood, 5 
Stead’s brittleness, 51 
Steam test, 163 
Steel, 88 

air hardening, 117 
alloy, 88-108 
Bessemer, 89 


INDEX 


200 


Steel, carbon, 88 
castings, 104 
cementation, 89 
cold bending, 99 
crucible, 89 

at high temperatures, 103 
impurities, 91 
manufacture, 88 
relative value, 92 
self hardening, 109 
specifications for, 106 
strength of, 96 
Stone, 134 
durability, 141 
elasticity, 136 
kinds, 134 
protection, 142 
resistance to fire, 142 
specifications, 142 
weight, 135 
Strength, brass, 129 
brick, 144 
piers, 146 
bronzes, 129 
cast iron, 76 

cements and mortars, 167 
concrete, 175 
duralumin, 130 
hollow tile blocks, 151 
lime mortar, 155 
malleable cast iron, 86 
steel, 96 
terra cotta, 151 
timber, 17-24 
wrought iron, 81 
Summerwood, 5 

T 

Temper colors, 51 
Tempering, 56 
Teredo, 34 
Terra cotta, 151 
Timber, decay, 31 
defects, 9 


Timber, definition, 4 
preservation, 37 
strength, 19 
Tin, 128 

Tobin bronze, 132 
Troostite, 51 
Tungsten steel, 116 

y 

Vanadium steel, 116 
Vicat needles, 160 
Voids, 166 

W 

Water hammer, 79 
Water proofing, 195 
Welding, 63 
Wire drawing, 61 
Wood, checks, 9, 24 
defects, 9 
definition, 4 
grain, 10 
heartwood, 5 
kinds of conifers, 4 
dicotyledons, 4 
used in structures, 15 
knots, 9, 24 
moisture, 6 
sapwood, 5 
seasoning, 8, 37 
shrinkage, 8 
slow burning, 38 
summerwood, 5 
wane, 9 
weight, 25 

Wrought iron manufacture, 80 
muck bars, 80 
strength, 81 
uses, 81 

Z 

Zinc, 127 

Zinc chloride (Burnettizing) 







